From Kaplan:
Example: Discrete uniform distribution

Determine p(6), F(6), and P(2 ≤ X ≤ 8) for the discrete uniform distribution function defined as:

X = {2, 4, 6, 8, 10}, p(x) = 0.2

Answer:

p(6) = 0.2, since p(x) = 0.2 for all x. F(6) = P(X ≤ 6) = np(x) = 3(0.2) = 0.6. Note that n = 3 since 6 is the third outcome in the range of possible outcomes.

What does F(6) stand for & how were the particulars used to find F(6) = 0.6 ?

F is called a cumulative distribution function: F(x)=P(X ≤ x) where X is a random variable and x is a specified constant.

In this case, F(6) means the probability of getting a value less than or equal to 6. This includes the values 2, 4, and 6. Since each value has a uniform probability of 0.2 of occurring, then F(6) = P(X≤ 6) = p(2) + p(4) + p(6) = 0.2 + 0.2 + 0.2 = 3 * 0.2 = 0.6