You might have heard of dialogue tags before. They're the little descriptors that go along with dialogue to further color and describe the dialogue, such as "he screamed" or "she moaned." Well, tags are a useful label for another phenomenon in creative writing. They are often unconsciously and habitually used when describing characters. The problem comes when character tags are too repetitive.
Most of our stories take place in populated worlds. In addition to our main characters, we've got relatives, police officers, shopkeepers, doctors, and repairmen running around, dropping into the story and just as quickly dropping out. We want to make them real, but not too real, robbing attention from the main character. So we tend to give them just one or two descriptors, noticing just one thing about them. We say that the flower lady has a red nose or that the policeman has a funny Adam's apple, or that the repairman keeps blowing his nose. Generally, this is consistent with the way we interact with people in the real world; they blur past us with just one or two characteristics sticking out. The girl with the yellow sweater, the man with the briefcase, the guy with the scar, etc. These characteristics are like memory tags, little mnemonic devices to help our readers keep the people straight.
However, in fiction this can begin to seem both reductive and boring. If every person we meet in the story is neatly paired with exactly one characteristic, it will begin to seem formulaic. Is the shopkeeper really just a blank persona with a red nose? Do we really only see the scar of the guy on the train? It begins to seem false and writerly, a creative writing convention. We have to keep shaking it up, and being very aware of habits or patterns that we're falling into.
So what sort of description do our characters deserve? Instead of giving them static appearance-based tags, give them small hints that they have their own stories and conflicts going on. Have the shopkeeper talking to her boyfriend on the phone and clearly in the middle of a fight. Have the guy on the train reading an interesting book. Let the man with the briefcase muttering and shaking his head as if he's had a bad meeting. Give your characters tags with backgrounds to them instead of still, unchanging tags. This will make the characters seem less like cardboard extras on a movie set, and more like people rushing off to tend to their own problematic worlds.