I am trying to understand this formula:

=INDEX(B5:C19,SMALL(IF(B5:B19=B21,ROW(B5:B19)-ROW(B5)+1,ROW(B19)+1),C21),2)

but am confused with the portion on

IF(B5:B19=B21,ROW(B5:B19)-ROW(B5)+1,ROW(B19)+1)


Can someone help to explain it please? Thank you very much.