As I am going over the uses of N5 kanji, I have gotten confused as to why 口（ぐち） is used in places such as 出口 and 入口. What meaning does it add?
As you are probably aware, 口 has the meaning of mouth. However, when you look at the Jisho.org definition, you will also find the definition of
opening; hole; gap; orifice as well.
What meaning does it add?
In this case, the meaning that is added is pretty simple, opening.
Combining 出 with 口, you get a hole for exiting. You will encounter this at many building and parking lot exits, but it exists many other places as well.
Combining 入 with 口, you get a hole for entering. You will encounter this at many building and parking lot entrances. As with 出口 you will also see it in other places as well.
Here 口 means "doorway" giving
- 入口 "entry" ("entering doorway")
- 出口 "exit" ("exiting doorway")
Of course the principal meaning of 口【くち】 is "mouth" — the opening through which humans or animals take in food. However, besides this literal meaning, "mouth" also has an extended meaning, also in English:
2 An opening or entrance to a hollow, concave, or enclosed structure.
‘the mouth of a cave’
A very similar meaning exists also in Japanese and a monolingual dictionary will list something like the following (taken from 大辞林):
㋐ 穴やすき間。 「傷－」 「船腹に－があく」
㋑ ものを出し入れする所。また、そこをふさぐもの。 「瓶の－」 「 －がかたくて抜けない」
㋒ 人の出入りする所。戸口。 「 －が狭い」 「登山－とざんぐち」 「非常－ひじようぐち」
roughly translating to
a space where it is possible to pass through; …
㋒ where people enter and exit; door or doorway
Note that there are many more such words — the cited dictionary entry gives the following examples:
- 非常口 "emergency exit"
- 登山口 meaning the start of an ascending mountain trail.