What does it mean if something is 'a steal'?

That means it's offered for sale at a very very very very good price.

The price may be so good because the owner doesn't know what it's actually worth,

or because they're having a super-duper temporary sale, or because it's something

you personally need very badly so it's worth a lot more to you personally.

In any case, it's called a 'steal', because when something is stolen, the stealer

gets what he wants for a very very very low price, and this is just like that.