Answer: It means worshiping Allah by abstaining from the invalidators of fasting from the break of dawn until sunset, while having the intention of fasting. It is of two types:

First: Obligatory fasting: such as fasting the month of Ramadān, which is one of the pillars of Islam.

Allah Almighty says: {O you who believe, fasting is prescribed upon you as it was prescribed upon those who were before you, so that you may become righteous.} [Surat al-Baqarah: 183]

Second: Voluntary fasting: such as fasting on Mondays and Thursdays; fasting three days every month, the best of which are the White Days (13th, 14th, 15th) of every lunar month.