Betting odds are the ratio between the amount staked by the bookies and the bettor, so 7/1 means the bookies stake seven times the amount the bettor has wagered. If the bettor wins; their predicted outcome materialises; they will take seven times their bet from the bookie (in this case).
Mathematicians measure probability by counting and using some very basic math, like addition and division. For example, you can add up the number of spades in a complete deck (13) and divide this by the total number of cards in the deck (52) to get the probability of randomly drawing a spade: 13 in 52, or 25 percent.