Salah (Prayer) is the most obligatory of the Five Pillars next to Shahadah ie, testification of Islam and it was ordained in the best and the most perfect way for an act of worship. Salah has been ordained 5 times a day for a Muslim. Among the five prayers is the Zuhr prayer which is the second prayer. 
The Literal Meaning of Zuhr/ الظهر can be:
ظهْر ( اسم ) : ذُو حَدَبَة
humpback ; hunchback
ظَهْر : سَطْح
face ; surface ; top ; upper part
ظَهْر : ظَهْر
tergum - back; rear part of the human or animal body from the neck to the buttocks.
ظَهْر : قَفاً
back ; hind (part) ; hinder part ; rear (part) ; reverse ; verso
ظُهْر : نصْفُ النّهَار
midday ; noon 
Its specified time begins when the Sun passes its meridian and declines westward, and this is indicated in the verse of Allah , when he says : “Establish prayer at the decline of the sun [from its meridian]”…Surah Al-Isra 17:78.
Such a decline of the sun is known when the shadow of an object becomes eastward instead of westward , and this specified time for the Zuhr prayer continues until an object’s shadow becomes approximately the same in length as the object itself and ends at this time, for the Prophet ﷺsaid : “The time for Zuhr is from when the sun has passed its zenith and a man’s shadow is equal in length to his height, until the time for ‘Asr comes. The time for ‘Asr lasts until the sun turns yellow. The time for Maghrib lasts until the twilight has faded. The time for ‘Isha’ lasts until midnight. The time for Subh (Fajr) prayer lasts from the beginning of the pre-dawn so long as the sun has not yet started to rise. When the sun starts to rise then stop praying, for it rises between the two horns of the Shaytaan.” Sahih Muslim, 612
It is desirable to hasten the Zuhr Prayer at the beginning of its specified time unless it is extremely hot, in which case it is desirable to delay performing until it is cooler, as the Prophet ﷺsaid: “In a very hot weather, delay the Zuhr Prayer until it becomes (a bit) cooler, because the severity of heat is from the raging of the Hell-fire”. Sahih Bukhari 538 
A stick or pole is put in an open place. When the sun rises in the east, the shadow of this stick will fall towards the west. The higher the sun rises, the shorter the shadow will become. So long as it keeps growing shorter, the sun has not yet reached its zenith. The shadow will keep on growing shorter until it reaches a certain point, then it will start to increase, falling towards the east. When it increases by even a small amount, then the sun has passed its zenith. At that point the time for Zuhr has begun.
Knowing the time of the zenith by the clock: the time between sunrise and sunset is divided in half, and that is the time of the zenith. If it is assumed that the sun rises at 6 a.m. and sets at 6 p.m., then the zenith is at 12 noon. If it rises at 7 a.m. and sets at 7 p.m., then the zenith is at 1 p.m., and so on. 
The end of the time for Zuhr is when the shadow of everything is equal in length to the object itself, plus the length of the shadow of the object at the time of the zenith.
Practical way of knowing when the time for Zuhr has ended: The stick or pole which is described above. If it is assumed that its length is one meter. It can be noticed that before the sun reached its zenith, the shadow decreased gradually until it reached a certain point (a mark should be made at this point), then it started to increase, at which point the time for Zuhr began.
The shadow will continue to increase, falling towards the east until the length of the shadow is equal to the length of the object itself, i.e., it will be one meter long, starting from the point marked at the zenith). As for the shadow before the mark, that is not counted, and it is called fay’ al-zawaal (the shadow of the zenith). At this point the time for Zuhr ends and the time for ‘Asr begins straight away. 
Narrated Anas bin Malik: He offered four rak`at of Zuhr prayer with the Prophet (ﷺ) at Medina. Sahih al-Bukhari 1089 
"Allaah will build a house in Heaven for whoever is diligent in observing 12 Sunnah Rak'aat (as follows): 4 Rak'aat before and 2 after the Zuhr (Midday) Prayer, 2 after the Maghrib (Sunset Prayer), 2 after the ‘Ishaa’ (Evening) Prayer and 2 before the Fajr (Dawn) Prayer." At-Tirmidhi No. 414 and Ibn Majah Book 5, Hadith 1194 AndHadith No. 6183 & 6362 in Sahih al-Jaami’ 
It was narrated from Mu’adh bin Jabal that the Prophet (ﷺ) combined the Zuhr and ‘Asr, and the Maghrib and ‘Isha’ when traveling during the campaign of Tabuk. Sunan Ibn Majah, Vol. 1, Book 5, Hadith 1070
Ibn Abbas said: "Allah'sMessengerﷺcombined the Zuhr and Asr (prayers), and the Maghrib and Isha (prayers) in Al-Madinah, without being in a state of fear, nor due to rain." Jami` at-Tirmidhi 187