There are lots of strange terms used in Matched Betting, here are some definitions.
This is where odds from different bookies overlap on the same event. So for example you can make a profit regardless of using any free bets. Arbs generally give a profit of up to 15% maximum but that can quickly add up. Profit Accumulator have an Oddschecker that gives a list of arbs you can bet on, updated throughout the day.
This is where you place a bet against the outcome, ie bet against a horse winning etc. In Matched Betting there are always two bets, one for the outcome and one against. Lay bets are usually place on a betting exchange such as Betfair.
This is where you place your "Lay" bet - a betting exchange such as Betfair allows you to bet directly against other bettors and so you can bet on pretty much any outcome, including a horse not winning etc. Vital for Matched Betting.