Heel Lift is what happens when your heel moves up within your boot. Sometimes called 'slock'. When Skiing or Snowboarding heel lift can lead to fatigue, pins and needles from over tightened boots, loss of confidence and even sores and blisters.
Having your heels 'locked in' is a massive confidence booster. It means you will feel more in control and basically be able to make the most of your time in the mountains (or in your local snowdome or dryslope!).
To find out if you have Heel Lift in your Ski or Snowboard boots (or any type of footwear) you can do this test: Put your boots on, stand with you feet flat on the ground, then bend your knees and roll onto the balls of your feet (the forefoot area). Whilst doing this your boots should come with your heel, if you can feel that your boots don't move with you heel then there is lack of grip from the boot/footwear and this could lead to problems.
If you have Heel Lift you might want to have a look at SHL™ Stop Heel Lift boot fitting pads.