Puns are jokes that use different meanings of the same or similar sounding words. English spelling allows different words with the same sound to be spelled differently, so there is more opportunity in English for puns. Puns are very difficult to understand unless you know both meanings of the words. Although there is an expression in English, “If you have to explain the joke, it is no longer funny,” I will try anyway.
When a clock is hungry it goes back four seconds.
The pun here is on the words seconds and four/for. Seconds has two meanings: a measure of time (60 seconds in 1 minute), and a second helping or serving of food. For example, if you are eating at home and your spouse asks you if you want “seconds,” s/he means “Do you want another serving of the food you just ate?” Four is a number (4), and for is a preposition. To go back for seconds means to go get more food. To go back four seconds means the clock reverses in time four seconds. So this is a “double pun,” in that we are “punning” on the words seconds and four.
Don’t worry if you don’t think the joke is funny. I liked it, but I really like puns!