Learn Icelandic: Við sjáumst bráðum! - We will see each other soon!

This is the middle voice form of the verb að sjá (to see). The active form við sjáum just means "we see", but adding the -st ending changes the meaning to a reciprocal action: "we see each other".

Why does the verb end in -mst here instead of just -st?

You are actually just adding -st to the regular, fully conjugated verb. The active form for "we" is við sjáum. When you attach -st directly to the end of sjáum, you get sjáumst.

Does adding -st to a verb always mean "each other"?

Not always! While it often creates a reciprocal action (doing something to each other) like in this sentence, the -st ending can also make a verb passive, or change its meaning entirely depending on the verb.

The English translation says "will see", but where is the word for "will"?

Icelandic does not have a separate future tense form for verbs. Instead, you simply use the present tense verb (sjáumst) along with a time-related word like bráðum (soon). The context tells us that the action is happening in the future.