Mustahab (مستحب) acts are those acts which are encouraged in Islam but there is no penalty/ sin for not doing them. They are a degree below farz (obligatory) acts.
(re)commendable ; agreeable ; chosen ; desirable ; desired ; favorite ; nice ; pleasant ; popular ; preferred ; sought after ; well-liked ; wished for 
Arabic : مُسْتَحَبّ:
The word mustahab refers to something that is enjoined in Islam but is not absolutely binding, such as Fasting on Mondays and Thursdays. The one who does something that is mustahab will be rewarded. 
Mustahab is a Fiqh or Islamic Jurisprudence term which means a Sunnah of the Messenger of Allah (ﷺ) which is highly recommended and desirable for the believers to follow; but if one does not do it, there is no sin upon the person. It means a deed which the Messenger of Allah (ﷺ) recommended the believers to do, but did not make it fardh or obligatory. 
Mandub (commendable), Mustahab (desirable), and Sunnah (whatever is reported from the Prophet) are similar, if not synonymous. All of these terms refer to a category of acts that if one performs, one will be rewarded, but if one does not perform, one will not be punished. 
Sunnah mu’akkadah (confirmed Sunnah), naafil (supererogatory), voluntary and mandoob/mustahabb (recommended) all share a similar meaning; they are acts of worship that are enjoined and encouraged in Islam, without being obligatory. The one who does them will be rewarded but there is no sin on the one who does not do them.