If something is in plain view, it's out in the open with no obstructions. But usually we use plain as an adjective to describe unembellished, bare, or unimpressive things.

Is plain a noun verb or adjective?

plain ​Definitions and Synonyms ​‌‌

​noun. UK /pleɪn/ plain adjective. plain adverb. plain old phrase.

Is plain an adjective or an adverb?

PLAIN (adverb) definition and synonyms | Macmillan Dictionary.