ANSWER:
Indexing is accessing your data faster; readings and updates in the database.
EXPLANATION:
Indexes allow you to look up a small subset of the fields called keys, which are stored in an ordered order, and find the actual data using the key. These keys in the index point to the actual data records.
Indexes also help in sort operations, if the sort is on an indexed field. Also, a single index can be useful in multiple queries.
Indexes take up storage space and, while they are running, memory. When data is created or updated, the indexes need updates, sometimes; that's extra write. Therefore, the use of indices needs careful study.