Has this ever happened to you?
The man on the left is trying to order shuǐjiǎo (dumplings) but he can’t remember what tones to use, so the vendor doesn’t understand him.
Along comes the new man on the right who uses the proper tones and thus gets what he wants.
To native English speakers, words sound the same even when you change the tones used, but to Chinese speakers they result in completely different words with completely different meanings.
So make sure when you’re learning new vocabulary in Chinese, to also make note of what tones to use to avoid being misunderstood!