it means 6:00 pm 1800 (or 18:00) is equivalent to 6:00pm If one counts from 12, it is 13:00 (1:00pm), 14:00 (2:00pm) and so on

Oh，you said it.Actually i saw this in an article about war.Thank you soooo much.

By the way, this is called "military time." It's common for military personnel to use a 24-hour clock instead of a 12-hour clock, since there's less chance of confusion.