Can someone tell me what this means exactly? Does this mean that a provider will rub or grind her wet pussy against your bare cock? Do a lot of providers do that?

No way to know how many do it.  Few will admit it, as it's basically BBFS.
That reminds me, where's that BBFS4Ever guy?

I don't know how many do it, but from what I've seen in reviews it's a fair percentage.

Yes, that happens. Some of them slide their whole bodies back and forth, some do precision slides, both simulating intercourse. Others manhandle you, and get it in there as close as possible to penetration without penetration.

There are lots of discussions on that. Make an educated decision or find the 'service' you may be looking for. Here, I'll make it easier for you:

Stripper Slide (BBFS Extra Lite):
https://www.theeroticreview.com/discussion_boards/viewmsg.asp?MessageID=878535&boardID=12&page=#878535

P2P:
https://www.theeroticreview.com/discussion_boards/viewmsg.asp?MessageID=887596&boardID=12&page=#887596

Bareback. Do you or don't you?:
https://www.theeroticreview.com/discussion_boards/viewmsg.asp?MessageID=878355&boardID=12&page=#878355