mother board vs add-in

mother board and add-in both are nouns.

Word	Noun	Adjective	Verb	Adverb
mother board	Yes	No	No	No
add-in	Yes	No	No	No

As nouns, add-in is a hypernym of mother board; that is, add-in is a word with a broader meaning than mother board:

mother board: the main circuit board for a computer
add-in: a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities

Other hypernyms of mother board include board, card, circuit board, circuit card, plug-in.

mother board (noun)	add-in (noun)
the main circuit board for a computer	a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities