Hash, Line Hashing, Simple MD5 Hashes - Look down below for the answers to your questions:
A hash function is a mathematical function that converts a larger range of data into a small piece of information, often only an integer.
This can serve as a key to an array. Other names for these hashes are: hash sum, hash codes or hash values
Hash tables are used to speed up lookups in larger tables of data, but sometimes hash functions can map two different set of datas to the same hash value.
You must not confuse hash values with checksums, which are a completely different thing.