Explanation of an array formula with SMALL()

I saw this post, which was solved using this formula
=IF(ROW(A1)>SUMPRODUCT(--(ISNUMBER(FIND("REQUEST ID",Sheet2!$A$1:$A$50000)))),"",INDEX(Sheet2!$A$1:$A$50000,SMALL(IF(ISNUMBER(FIND("REQUEST ID",Sheet2!$A$1:$A$50000)),ROW(Sheet2!$A$1:$A$50000)),ROW(A1))))

I'm trying to understand how does it work, or what does each part is doing but am currently stuck. Could anyone explain what is going on, particularly with the SMALL ()?

Thanks!