How are bitmap indexes helpful?

Wikipedia gives this example

Identifier    Gender         Bitmaps
                              F    M
1           Female            1    0
2           Male              0    1
3           Male              0    1
4           Unspecified       0    0
5           Female            1    0

But I do not understand this.

• How is this an index first of all? Isn't an index supposed to point to rows (using rowid's) given the key?
• What would be the typical queries where such indexes would be useful? How are they better than B-tree indexes? I know that if we use a B-tree index on Gender here, we will get a lot of results if for example, we look for Gender = Male, which need to be filtered out further (so not very useful). How does a Bitmap improve the situation?