Mapping functions and Collision

Mapping functions and collision are not clear. Can you please in a simple term

A mapping function is used to map the values into the table.
And collisions are the situations where a hash function generates a same key value again. Hash functions that maps most values to unique locations are more suitable / acceptable. And they provide minimal number of collutions (Cases inwhich two values map to the same location). Chaining is the easiest way to handle colluisions.