Bitmap indexes consist of a bitmap (bit vector) for each distinct column value. Each bitmap has one bit for every row in the table. The bit is on if the related row has the value represented by the bitmap.
The advantage of bit map indexes is that complex queries can be processed by performing Boolean operations (AND, OR) on the indexes – efficiently determining exactly which instances (rows) fit the query conditions, without searching through the entire database. For complex queries, bit-map indexes can boost response times by more than a factor of 100.
From Wikipedia: Bitmap indexes have traditionally been considered to work well for data such as Boolean values, which have a modest number of distinct values – in this case, boolean True and False – but many occurrences of those values. This would happen if, for example, you had data on whether or not each resident in a city has internet access.