The 24-hour clock is a way to tell time that uses all the hours of the day, instead of just the 12 hours that we usually use.
You know how you learn about the clock and how there's a big hand and a little hand? Well, the big hand shows the minutes and the little hand shows the hours. When we use the 24-hour clock, we go from 00:00 (which means midnight) to 23:59 (which means the last minute before midnight again).
So, let's say you want to know what time it is in the afternoon. With the 12-hour clock, you might see that it's 3:45 PM, but with the 24-hour clock, you would read it as 15:45 (because 3 PM is the 15th hour of the day).
Using the 24-hour clock can be helpful because it avoids confusion about whether you mean 3 AM or 3 PM. It's especially popular in places like the military and hospitals where it's important to have precise timing. But don't worry, you don't have to use it all the time. Just be aware that it exists and might come in handy someday!