
reingehen
to go in
To enter a room, a building, or a defined area. It is the colloquial form of 'hineingehen' (to go in / into).
Es ist kalt draußen, lasst uns schnell wieder reingehen.
It's cold outside; let's quickly go back in.
to go in
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to go in
To enter a room, a building, or a defined area. It is the colloquial form of 'hineingehen' (to go in / into).
Es ist kalt draußen, lasst uns schnell wieder reingehen.
It's cold outside; let's quickly go back in.

to fit in
To fit into something, whether because of size or volume.
Glaubst du, der ganze Einkauf geht in den kleinen Kühlschrank rein?
Do you think all the groceries will fit in the small refrigerator?

to sink in
Colloquial for understanding or grasping information. It is often used in the negative form to express that someone does not understand something.
Ich habe ihm die Regeln erklärt, aber es geht einfach nicht in seinen Kopf rein.
I explained the rules to him, but it just won't sink into his head.
'Reingehen' (to go in) is a separable verb. The prefix 'rein' stands at the end in a main clause. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Kommt, wir gehen jetzt rein.
Come on, we're going in now.
The prefix 'rein-' is the colloquial short form of 'herein-' or 'hinein-'. It expresses movement into a place and is used very frequently in spoken language.
Darf ich kurz reinkommen?
May I come in for a moment?
In the meaning 'hineinpassen' (to fit in), the structure 'etwas geht in etwas rein' (something fits into something) is often used. The subject is the thing that is supposed to fit, and the destination is indicated with the preposition 'in' (+ accusative).
Geht der große Koffer auch noch ins Auto rein?
Will the big suitcase still fit in the car too?
The figurative meaning 'verstehen' (to understand) is almost always used in the fixed expression 'etwas geht (nicht) in den Kopf rein' (something does / does not go into one's head). The word 'Kopf' (head) is the key clue for this meaning here.
Diese komplizierte Grammatikregel geht einfach nicht in meinen Kopf rein.
This complicated grammar rule just won't sink into my head.
to understand
Both words can say that information is not understood. Verstehen is neutral and suitable in any standard situation. Reingehen is colloquial and is used especially in negative statements such as “Das geht nicht rein” (That isn't sinking in).

Die neue Regel geht bei mir noch nicht rein, deshalb lese ich die Erklärung noch einmal.
The new rule still isn't sinking in for me, so I'm reading the explanation again.
Ich verstehe die neue Regel noch nicht und lese die Erklärung deshalb noch einmal.
I still don't understand the new rule, so I'm reading the explanation again.