Starting at 0 we can count up in units like this:
0 1 2 3 4 5 6 7 ...
But what happens before zero?
We have the negative numbers, like this:
... -7 -6 -5 -4 -3 -2 -1 0
That was counting UP (the numbers are increasing)
Now let's count BACKWARDS from 7 (the numbers are decreasing)
7 6 5 4 3 2 1 0 -1 -2 -3 -4 -5 -6 -7 ...
Example
Count backwards to work out the missing number.
4 3 2 1 0 -1 -3
What is the missing number?
Answer
Counting backwards from 0, it goes:
... 0 -1 -2 -3
So the missing number is -2.