The Y-axis on the CDF graph represents P(X ≤ x)
The CDF always goes from 0 to 1 (because you cannot have less than 0% chance of getting something and no more than 100%)
Well, let's take the classic coin toss as a discrete example:
You flip the coin twice. You count the number of heads (i.e. heads = 1, tails = 0)
You have four possibilities:
0+0 = 0
0+1 = 1
1+0 = 1
1+1 = 2
I guess you know that the probability of getting any of these specific outcomes is equal to 1/4. The probability of getting P(X ≤ 0) = 1/4 (i.e. only 0+0)
The probability of getting P(X ≤ 1) is 3/4. Why? Because three of the possibilities above are equal to or less than 1 (0+0, 0+1 and 1+0).
P(X ≤ 2) = 1 because all of the possibilities above are equal to or less than 2..