Please explain the use of suffix.

I don´t know this well. But it is 2 suffixes: + ma + sı

gitmesi - his/her/its going

This link mentions this pattern but does not explain it:

http://www.turkishlanguage.co.uk/infinitive.htm

Ex:

Herkesin bu kitabı okuması gerekiyor.

Everyone needs to read this book.