The RLI, or right lifted increase, is a single increase that creates two stitches from one.
To work it, insert your right-hand needle into the stitch below the first stitch on your left-hand needle from back to front. Lift just this stitch and place it, untwisted, onto your left-hand needle. Knit this lifted stitch, then knit the one following it. If you find this increase is creating small holes and you’d prefer them closed, knit the lifted stitch through the back loop.
TECHknitting has a very clearly illustrated post about how to work this increase.
The following video demonstrates how to work a RLI: