Like the name implies, a strap-on dildo is a dildo one can strap on. It’s a sex toy used to penetrate a lover.
A lesbian can use it to penetrate her lover vaginally or anally, a heterosexual girl can use it to penetrate her boyfriend from behind and a guy can use it if he fails to get a hard on.
If your lover wants to feel something in the vagina or anus during sex, fingers or a dildo might do. But the advantage of a strap-on dildo is that you can penetrate your lover and still have your hands free to caress the thighs, breasts, nipples, clitoris or any other body part.
There are various models. A strap-on dildo can be permanently attached to a leather or latex panty. But it can also be removable: a harness to which you attach the dildo yourself.
Before you buy anything think about the material you would like, about the size the dildo should have and about the angle in which it will be attached. When it points skyward it’s good for g-spot stimulation in the missionary position, while one that points downward works better with anal penetration. So you have to know beforehand what you want to do with it.

How to use it well?
A thigh harness