Certain Icelandic verbs have another form that ends in -st, such as “að komast”, “að giftast”, and “að sjást”. If you look the inflection tables of these words you will see them labeled as the “middle voice” (miðmynd).
A verb having -st usually indicates one of the following:
  • that something is becoming done
  • that something is able to be done
  • that something is being done to itself
  • that the subjects are doing something to each other
Examples:
  • „Kakan er að bakast“ (The cake is being baked). Shows that the thing in the process of being done.
  • „Við sjáumst á morgun“ (We will see each other tomorrow). Shows that the two things will do something to each other.
  • „Við sjáumst ekki í myrkrinu“ (We cannot be seen in the dark). Discusses whether something is able to be done.
  • „Ég kemst í gömlu buxurnar mínar“ (I am able to fit into my old pants). Discusses whether something is able to be done.
  • „Ég kemst ekki í afmælið þitt“ (I am not able to make it to your birthday party). Discusses whether something is able to be done.
  • „Þau kysstust þegar þau sáust“ (They kissed each other when they saw each other). Shows that the two people do something to each other.
  • „Hvað er að gerast?“ (What is happening?) Shows that the thing in the process of being done.

Note that there may be ambiguity about whether something is actually an -st variant or just a form that happens to end in -st in one particular case but not all the related cases. See here:
  • ég kom í gær (I came yesterday), þú komst í gær (you came yesterday), hann kom í gær (he came yesterday). This is just the regular verb in the regular voice despite the fact that one case, second person singular past tense, does happen to end in -st. This usually only happens with second person singular past tense.
  • ég komst í gær (I was able to come yesterday), þú komst í gær (you were able to come yesterday), hann komst í gær (he was able to come yesterday). This is truly the -st form of the verb, all of the related words end in -st.
This ambiguity is very rare, and you can almost always figure out what is meant from context.

Notes
You might be interested in knowing why these words end in -st, but if you’re not you can ignore this section. The explanation is as follows:
In English, you can say: “He calls himself John”. The same can be done in Icelandic: “Hann kallar sig Jón”. “Himself” and “sig” are called reflexive because they imply something is happening to itself. A very long time ago, the sig was added to the verb, and that slowly became -st. The meaning also changed a bit.
  • Hann kallar sig Jón. → Hann kallast Jón.
  • He calls himself John. → He is called John.
  • Hún klæddi sig. → Hún klæddist.
  • She dressed herself. → She got dressed.
  • Hann brenndi sig. → Hann brenndist.
  • He burned himself. → He got burned.