It is a two- raka’at Salat prescribed upon anybody who enters the mosque, before he sits down.

Its ruling

The evidence for this is taken from the statement of the Prophet ﷺ: “When any of you enters a mosque, he should perform two rak’ahs before sitting down.” [ Agreed upon.]

However, the routine supererogatory Salats observed at the time for obligatory https://www.al-feqh.com/en/supererogatory-salat" rel="nofollow - Salats (i.e Ar- rawaatib), or even the obligatory Salat itself, can stand in its place. Thus, if one enters the mosque to observe Salatu Dhuhr, for example, and (just after he enters) he observes the supererogatory Salat before Dhuhr, he is not required to observe Salatu tahiyyatul Masjid anymore, because the supererogatory Salat has stood in its place.