What does it mean when you have a solid dot on your graph?

It usually means that the end point is included in the range of values that you are intereted in.

For example, a graph of 0 ≤ x < 1 would have a solid dot at 0 and a hollow circle at 1

while a graph of 0 < x ≤ 1 would have a hollow circle at 0 and a solid dot at 1.