Noun: Exterior door, external door, outside door.
Ovi is a noun that means door.
Ulko- is a prefix used to signify ”outer,” ”outside,” or ”external.” It is also used to describe people or things from ”overseas” or ”living outside one’s country.”
Ulko– appears in many common compound words. It’s used with a dash in ulko-ovi only so that the vowel does not double into ”oo” in the middle. In most compound words made with ulko- there is no dash.
Let’s translate the rest of the sign, ”pidettävä suljettuna,” which means ”must be kept closed.”
Pidettäva is the passive present participle of the verb pitää (see pitää fully conjugated here). Pitää has dozens of possible meanings depending on use. Check some of them on the Sanakirja.org page here.
One use of passive present participle is to denote something that has to be done. In our example, pitää means ”to keep” and pidettäva best translates as ”has to be kept”, ”should be kept” or ”must be kept.”
Suljettuna is the essive singular of suljettu, which means ”closed.” Essive case in Finnish can have the meaning of a temporary location or state of being (i.e., lapsena, essive of lapsi (”child”) means ”as a child”), or can an answer to the question ”how” or ”in what way.”