your right, it's an extra attack. You only get the extra attacks on a full-attack however.
When you reach +6 BAB, you get another attack at -5 (aka +6/+1.)
When you reach +11 BAB, you get yet another one at -10 (aka +11/+6/+1)
When you reach +16 BAB, you get the last one at -15 (aka +16/+11/+6/+1)
If you ever get to +21, you don't get any more attacks (unless homerule) so it will be +21/+16/+11/+6.
Yes, that's what it means.
For every 5 points of BAB above 1, a character gets an additional attack when he makes a full-attack action (also known as iterative attacks).
A character with BAB +6 will have two attacks at +6 and +1.
A character with BAB +11 will have three attacks; at +11, +6, and +1.
A character with BAB +16 will have four attacks; at +16, +11, +6, and +1.
There are no additional iterative attacks for BAB +21 or higher.