Bet level is the number of coins you stake per line, or per spin on games without lines. Multiplied by the coin value and the number of lines, it gives the total bet.
Illustrative example: on a 10-line game with a 10p coin value, bet level 1 costs £1 a spin and bet level 3 costs £3. Raising the level scales all line wins in proportion.
On almost all modern games, bet level does not change the RTP or unlock better odds; it only changes the stake. A few older machine-style games paid a bonus or jackpot only at maximum level, which the paytable would state. GB online stake limits apply to the resulting total bet.