They have very limited power these days. They advise the House of Commons, as more or less a hereditary think tank. They can delay the passage of bills, though this has been limited to a maximum delay of one year since 1949 (less for some types of bills) and there are some checks on this ability. They have a few other things they can do that are (IMO) too boring to warrant much thought unless you're a member of parliament.

The idea of a House of Lords does strike me as a bit odd, but it's not really the big deal it used to be.