Sometimes when we use more than one word as an adjective we need to join those words with a hyphen to show they belong together and make our meaning clear.
For example if we say “man-eating tiger” the hyphen helps us to know that the tiger eats people. If we leave it out—”man eating tiger”—it could mean completely the opposite.
It’s not always the case that the meaning changes so dramatically if you leave out the hyphen, but including hyphens in the right places does help the reader and improves clarity. Examples include “virus-associated clinical signs”, “antigen-stimulated cytokine release”, “domestic short-haired cat”. And if you are talking about the dog pictured above, you may want to talk about a short dog wearing a coat—”a short, coated dog”—or a dog wearing a short coat—”a short-coated dog”.
It can be hard to know when you need a hyphen and when you don’t. This short video by Uncle Mike from soisitjustme gives some good examples and a quick trick to help tell.