Hajj is the symbol of Tawheed (the Oneness of Allah) from the first moment the pilgrim enters ihraam. Jaabiribn ‘Abd-Allah said, describing the Hajj of the Prophet Muhammad (May Allah honour Him and grant Him peace): “Then he stared to say the words of Tawheed, ‘“Labbayk Allahumma labbayk, labbayk laashareeka lakalabbayk. Inna al-hamdwa’l-ni’mata lakawa’l-mulk, laashareeka laka (Here I am, O Allah, here I am. Here I am, You have no partner, here I am. Verily all praise and blessings are Yours, and all sovereignty, You have no partner).’” Sahih Al Muslim
This trains the soul to acknowledge the Oneness of Allah (Glory be to Him) and to be sincere towards Him. The pilgrim starts his Hajj with Tawheed, and continues to recite the Talbiyyah with its words of Tawheed, and he moves from one action to the next with Tawheed.
It has many meanings few of them are
“Labbayka Allahumma labbayk (Here I am, O Allah, here I am)” is one response after another, which is repeated to show that the response is lasting and ongoing.
One of the meanings of the Talbiyyah is confirming love of Allah (Glory be to Him). There is an Arabic phrase imra’ah labbah (a loving woman) which refers to a woman who loves her child. One only says “Labbayk (Here I am at your service)” to a person whom one loves and respects.
It implies sincerity, as in the phrase lubb al-shay’ which means the essence of a thing, and lubb al-rajul which means a man’s mind and heart.
It may mean staying in one place (labba bi’l-makaan) and clinging to it, which means, I am persisting in obeying You, thus emphasizing the meaning of continuous servitude to Allah (Glory be to Him).
It is a symbol of the Tawheed of the religion of Prophet Ibrahim (Peace be upon Him), which is the spirit and aim of Hajj, indeed the spirit and aim of all the acts of worship. Hence the Talbiyyah is the key to this act of worship that the pilgrim is embarking on.
Prophet Muhammad (May Allah honour Him and grant Him peace) said: “Jibril came to me and ordered me to ordered my companions and those with me to raise their voices with Talbiyyah.” And he said: “the best Hajj is shouting out and spilling blood. Therefore the companions of the Prophet Muhammad (May Allah honour Him and grant Him peace) when they entered into ihram, by the time they reached `ar-Rauhaa' their voices had tired out. And the Prophet Muhammad (May Allah honour Him and grant Him peace) said: “It is as if I am looking at Prophet Musa (Peace be upon Him) , descending from the mountain pass raising his voice with Talbiyyah…”
And women, as regards the Talbiyyah, are like men - as the two preceding hadith are general - so they should raise their voices as long as there is no fear of fitnah. And `Aa'ishah used to raise her voice until the men could hear her.
And he may also recite along with it “La ilaha illallah” and Talbiyyah according to the saying of Ibn Mas'ud (R) - “I went out the prophet of Allah and he did not leave off the Talbiyyah until he stoned Jamrat-ul' Aqabah”.
Guide to hajj by 'Abdul-'Azlz bin 'Abdullah bin Baz