Any experienced database developer should be able to answer this question with ease. Some of the less-experienced developers will be able to answer it, but with a little less clarity.
In its most simple terms, an index is a data structure used to provide quick access to data in a database table or view. In SQL Server, they come in two flavors: clustered and non-clustered. Clustered indexes store the data at the leaf level of the index. This means that whichever field(s) in your table are included in the clustered index, they will be stored in an orderly fashion in the table. Because of this sorting, you can only have one clustered index per table. Non-clustered indexes contain a row identifier at the leaf level of the index. This row identifier is a pointer to a location of the data on the disk. This allows you to have more than one non-clustered index per table.