English can be a very tricky language. Even when you look at words that are spelled using the exact same letters, a hyphen can completely change the meaning. “Recreation” refers to leisure activity, whereas “re-creation” refers to creating something again. And the deal with “second hand” and “second-hand” works in much the same way.
“Second hand” (with a space) refers to the hand on a clock indicating the number of seconds in the current minute. It’s a noun. The ticking sound of a clock comes from the second hand. Its corresponding partners on an analog clock are the hour hand and the minute hand, pointing toward the hour and minute, respectively.
It’s a term that is also used figuratively, as would be the case with second-hand knowledge or second-hand experience. If you didn’t actually experience something yourself and simply heard about it from someone else, you have second-hand experience.
“Secondhand” (as a single word) is an alternate spelling of “second-hand.” Whether you use the hyphen or combine the words together into a single compound word is a matter of convention and preference. While some resources, like Wikipedia, seem to prefer secondhand, a Google search for second-hand yields more results. Go figure. But both secondhand and second-hand are distinct from second hand.