What is index exactly?

When you look something up in a book, you would look at the index at the back. It records all the words and a direct link to the page the word is on. An index in a database does the same, it stores all the data in the field and a direct link to the record. As this index is quick to search (it is in order) it makes finding records quicker. It means the database doesn't have to search through every record to find something. However, there is a cost to do the index. It takes memory and time to create and maintain. This means you don't want to index every field, just those you are most likely to need to refer to quickly. This is usually any I'd fields.