I wrote a SUMIF formula as follows and got an error
=SUMIF(C5:C10,=25,D5:D10)

but it worked when I put the quotes in the criteria like so:
=SUMIF(C5:C10,"=25",D5:D10)

Q: When do I use the quotes?
.................................................. .................................................. ......

It gets more confusing when I use a cell reference for the value 25 in the
criteria.

=SUMIF(C5:C10,"=B5",D5:D10) does not work - returns a value 0

=SUMIF(C5:C10,"="B5,D5:D10) Gives and error message

However
=SUMIF(C5:C10,"="&B5,D5:D10) works.

Q: Where do we put the quotes and what does the & infront of B5 do to make
it work?

I tired looking up & in the help menu but cant seem to find any references.