The answer to the riddle, "What travels the world but stays in the corner," is a stamp. This is because a stamp will travel all over the world on the mail it is attached to, but a stamp is still always placed in the corner of an envelope.
The riddle, like all riddles, is a question designed to trick people into over-thinking the answer. It requires ingenuity to find the answer or the meaning. There are many other riddles, as well.
Another example of a riddle is "What has a foot but no legs?" In this riddle, the answer is a snail.
Here is another example of a riddle that people commonly use: Poor people have it, rich people need it and if you eat, you'll die - what is it? The answer to this riddle is nothing. This is a great example of how riddles can have trick questions or, in this case especially, require some ingenuity to come up with the answer.
People, when trying to solve a riddle, commonly get stuck and can't find the answer because they aren't thinking about the riddle's question the right way. This is why riddles can be very tricky and many people enjoy trying to solve them or make up new ones.