When someone says to nip it in the bud the saying nip it in the bud basically means to stop something before it happens.
For example your kid is becoming a thief you would then want to nip it in the bud by stopping your child from becoming a thief.
The saying Nip it in the bud comes from the action of nipping a flower bud before it leads to the flower dying our going bad further.
So you would nip the bad flower bud before the flower completely died.
So to nip something in the bud means you would stop something from happening before it actually happens.