The causative form

せる is a "causative" helper verb form. It means the action of making or letting someone do something. Yes, it's used for both forcing people and allowing people.

For godan verbs, せる attaches to the negative stem. For ichidan verbs, さ is inserted between せる and the stem:

終わらせる
To make (something) end

死なせる
To let (someone) die

食べさせる
To make (someone) eat

When we use a verb in causative form, the causer/agent (= the one that does the “forcing” or “allowing”) is usually marked by は or が. The causee/target (= the one that is “forced” or “allowed”) can be either marked with に or を. The choice between these two particles depends on the verb being used.

When we use に, the interpretation of the causative is that of “allow” rather than “force”. On the other hand, when we use を, it could be either of them.

With intransitive verbs, the target can be marked by を, but with transitive ones the target can only be marked by に. In this type of usage, with transitive verbs, it is sometimes hard to tell if the sentence means “to allow” or “to force”.

子供を外で遊ばせる。
To let the kids play outside.

子供に飲ませる酒はない。
I won't make/let a child drink alcohol.

The causative form can also be chained with the passive (せる -> せられる).

書く to write
書かせる to make (someone) write
書かせられる to be made to write (by someone)

Causative-passive sentences cannot assume the meaning of “to be allowed” but only “to be forced”.

When used with する verbs, the causative form can turn an intransitive する verb, into a transitive one (する -> させる).

シミュレーターが起動する。
The simulator starts up. (intransitive)

カメラアプリを起動させる。
I start up the camera app. (transitive)