What is the consequence of not having a good hashing function?

A good hash function maps the key value pair with minimal collisions. When the hash function is not good there are frequent collisions in the mapping of the key value pair.

Collision means that if there is a key present in the container already with some value.If the hash function again maps the same key with different value at the same location in the container that previous key was present.

it can either be 2. or 4. : ) tell me if i'm wrong or right