Suffix allows you to place text at the end of the filename.

For example, if you had a series of files called:

test1.pdf
test2.pdf
test3.pdf

and you wanted them to all have summer_1965 on the end of the filename, you could type that into the Suffix field and the filenames would change to:

test1summer_1965.pdf
test2summer_1965.pdf
test3summer_1965.pdf