Mappings are used for a key to value mapping. This is a way to associate a value with a key. All values in this map are already initialized with all zeroes, for example, the following:
mapping (address => uint) offers;
This example shows that offers is declared as a mapping. Another example makes this clearer:
mapping (string => uint) bids; bids["packt"] = 10;
This is basically a dictionary or a hash table where string values are mapped to integer values. The mapping named bids has string packt mapped to value 10.
