Total bet is what one spin actually costs you. It covers every active payline or way, plus any optional extra such as an ante bet.
On older or line-based games it is calculated from components: coin value × coins per line (bet level) × number of lines. Illustrative example: a 1p coin, 5 coins per line and 20 lines gives a total bet of £1. Modern games usually just show the total.
Total bet is the figure that matters for working out cost, for comparing a win with what you staked (see losses disguised as wins), and for the GB online stake limits, which apply to the total for each game cycle. Many wins in the paytable are expressed as multiples of it.