Imagine you have a clock with numbers from 1 to 12 on it that tells you what time it is. The big hand on the clock goes all the way around in 60 minutes, or one hour. When the big hand is pointing at the 12, it means it's the beginning of the hour, and when it's pointing at the 6, it means it's been exactly 30 minutes since the start of the hour.
Now, when the big hand is almost at the 12 and just about to reach it, the time on the clock is 14:59. This is a way of writing down the time in a special way so we all know what we're talking about. 14:59 means that it's the 59th minute of the 14th hour in the day.
So basically, when you see 14:59 on a clock, it means you have 1 minute left until the next hour starts, and the clock will change to 15:00. It's important to keep track of the time, so you can plan your day and know when it's time for certain things like eating lunch or going to bed.