Consider some numbers in order on a number line:
-12 -11 -10 -9 -8 ... -2 -1 0 1 2 3 ... 8 9 10
Before means more negative than. Before -10 means to the left of -10 on the number line. -11 is more negative, less positive, than -10. But when we have positive numbers 9 is to the left of 10 because it’s less positive, meaning more negative. Think of a thermometer. -5 degrees is colder than -1; more to the left in temperature means colder, more to the right means warmer, or less cold.
-10.1 is also less than, or before, -10 because it’s more negative, and lies between -11 and -10.