=LET(name1, value1, [name2, value2, ...], calculation) gives names to values inside one formula, then uses them in the last argument:
=LET(over, B2-C2, IF(over>0, over*0.09, 0))
Without LET, B2-C2 would appear twice. With it, the formula says what each piece is, and Excel works it out once.
The names live only inside that formula; they don't appear in the Name Manager. Like other names they can't look like cell addresses, so use words such as sales or over, not x1. The last argument is always the result.