I am trying to value from two separate cells, but cannot figure it out. It
is similar to the "VLOOKUP" function, but that seems to only work for one
cell.

When I enter a description in "C4" and a thickness in "D5", I want to use
both of these information to to extract one number from another workbook.

Any Ideas?

You can concatenate the first argument in the VLOOKUP formula like
this:
=VLOOKUP(C4&D5,your range, column, range_lookup) but it's CRITICAL that
the concatenation matches the items in the lookup range (because that's
how VLOOKUP works). For instance if your description is "two by four
board" and thickness is "1.5 in" and you concatenate those, the result
is "two by four board1.5 in". This will NOT match if the lookup range
value "two by four board 1.5 in" because there are two spaces between
"board" and "1.5". You may need to modify that concatenated value so
it can match the lookup range.