What does it typically mean? How and when should I ask about it?

she's offering a discount from her normal rate.  The downside is, many providers will advertise specials and have no rates posted so you really don't know if you're getting a deal or not.

To ask about it, you could do that when you are contacting her for a date.  I would also say you should tell her where you saw it advertised.

Swi

In my experience, Specials are often for lesser time or lesser services.  The Special gets you in the door.  Be prepared to pay full rate or walk.