A pocket watch is like a special clock that you can carry around with you in your pocket. It has a little chain so you don't lose it.
When you want to know what time it is, you can take it out of your pocket and look at the front. You'll see two little hands that move around in circles. One of them is short and one of them is long.
The short hand points to the hour and the long hand shows the minutes. You might have to count the numbers to figure out exactly what time it is.
A long time ago, people used pocket watches all the time because they didn't have phones or other gadgets to tell the time. Nowadays, pocket watches are more like special things that people collect or wear as fancy accessories.