The house edge is the portion of every pound staked that a slot is designed to keep in the long run. It is simply 100% minus the RTP, so a 96% RTP game has a 4% house edge.
It works on total stakes, not on your starting balance. That matters because the same money is usually staked again and again. Illustrative example: start with £50, play £0.50 spins, and after 400 spins you have staked £200 even if your balance never went much above £50. At a 4% edge the expected loss on that £200 is £8.
No betting pattern, stake size or timing changes the edge. Every spin is settled by the RNG at the same odds, which is why slots cannot be "beaten" over time. The edge is how the game, the studio and the casino are paid. For the full picture of what you can and cannot control, see our safer gambling page.