Faisalk,

A tap is a tool used to create an internal thread in a hole, so a bolt turned into that hole will engage and hold.

What you were told is to get a larger tap - not too big for the housing, to clean out the remains of the old threads and to make a new, clean, one-size-larger thread.

When going to a hardware store, or a metal machining factory, you should take the engine crankcase and the old backplate bolt.
It is advisable to use a tap with a larger diameter, but the same pitch. I.e. if the bolt is 3mm-x0.5, tap the hole 4mm-x0.5.

You will need to use the correct bolt in that hole, not the original piece.

An alternative, cheaper fix is to use JB-Weld in the hole.
If the bolt is not tightened to strongly, it could hold.

In the future, be careful not to cross-thread the bolt and not to overtighten it.

Sincerely,