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.

'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
'verstehen' and 'reingehen' can both express that information reaches the mind. 'verstehen' is neutral and suitable in any situation. 'reingehen' is colloquial and is often used in the negative, for example when someone says that an explanation simply will not sink in. It sounds more personal and a bit more casual. For school, work, or clear explanations, 'verstehen' is usually better. 'Reingehen' fits more in casual conversation when you want to emphasize that something is not becoming understandable despite repetition.

Die technische Erklärung geht Ida heute einfach nicht rein.
The technical explanation just is not sinking in for Ida today.
In der Pause verstehe ich die neue Regel noch einmal in Ruhe.
During the break, I take another quiet moment to understand the new rule.