This convention, devised by William Flannery, uses an opening bid of 2 to show:

11-15 HCP with 4 and 5 (there are slight variations, but this is the gist of it).

The purpose of the convention is to avoid the awkward rebid problem when opening 1 and also holding a 4-card spade suit. The idea is to get the spade suit in without fear of getting involved with a reverse auction. The downside is the loss of a 2 opening for other purposes (such as a weak-2 bid).

Personally, I'm not a fan of this convention, but it is popular with maybe 25-30% of the expert community using it.