What does an open circle signify on a number line?

A blank dot when expressing a range for a number line indicates that we do not include the value in that range.

For example, this number line with a non-coloured dot on the 2 indicates not to include the number 2, so the numbers in this range are 3, 4, and 5

Whereas the number line below indicates the range 2 to 5, including the numbers 2 and 5, so the numbers 2, 3, 4, and 5