a squib is a non-magical person born in a magical family. they're simply muggles but the fact that they come from a witch and a wizard is unexpected.
if that was what you were asking...
A squib is sort of the reverse of a muggle-born wizard. They're a muggle born into a magical family.
A squib is a small, tailless cat, native only to the Isle of Man.
To answer your question, yes- they are an actual thing. As others have said, they're a person born into a wizarding family who doesn't posses magical powers.