Barrida means 'swept', and this is a move where one partner's foot sweep or drag the other partner's foot.
Why 'appears to'? Because in my prefered barrida method, the drag is illusory.
Here they are:
Barrida method 1
It's easiest to explain this using leader-to-follower barridas – ie, barridas where the leader's the one doing the dragging.
In this method, the leader leads the follower to stop slightly mid-step. Not right in the middle, but just before she completes the step, so that almost all her weight is on the leg she's stepping to, but a little bit is on the leg she's stepping from (at this point, what he's done is a parada). He then uses his foot to push her almost-free foot wherever he wants.
Here's a video from the Casas which demonstrates this approach.
I don't personally like this approach. The main reason I don't like it is because I feel it's very hard on the follower. See, followers have to maintain their balance very precisely – because they never know where they're about to go, they have to be perfectly balanced at every single moment – and anything that interferes with that makes their life difficult. If the follower has any weight on her 'free' foot, as she does in this method, dragging it is going to mess with her balance.
It's especially hard on her when she's asked to return the barrida using this method, because in that case, she's being asked to take a step, but an obstacle (his foot) is put in her way which she has to push out of the way in order to take the step she's being asked to take. If she follows the lead she's getting from the chest, she will trip over his foot. In order to gain the force to move his foot, she has to keep her weight on her standing leg till she's moved his foot out of the way enough for her to take the step she's being asked to take. And that means that she can't actually take the step she's being asked to take at the time she's being asked to take it – and that little delay between lead and following feels horrible for a follower. It breaks every rule her body's spent years learning.
So, I prefer method 2.
Barrida method 2
In barrida method 2, there is no actual dragging of the foot. Instead, the leader gives the lead from his chest, as he would for any other step. He leads the follower to take a step, and then as her foot moves to take that step, he just shadows her foot with his (making only the lightest contact, creating the illusion of dragging.
For example, if he leading her in a giro, then as he leads her to take a front or side step he could move his free foot along with hers.
Here's a video which shows this nicely – the follower is doing a giro around the leader, and he periodically inserts a barrida, completely without force.
This method makes follower-to-leader barridas easy as pie, because she doesn't have to exert any force to take her step, she can just step in the usual, natural way, while he just pretends she's moving him.
Barridas can go in all kinds of places
Here's a video showing how you can pop a barrida in almost any step: