in means “in” in English.
The preposition in is in the group of preposition that can be accusative or dative, depending on the meaning of the clause.
It must be emphasized again that "in" is as a "Wechselpräposition". This means that is can take accusative or dative depending on the clause.
Grammar:
In takes dative if the prepositional complement of place does not imply movement (location).
Ich bin im Kino
I’m in the theater
In takes accusative if the prepositional complement of place implies movement (direction).
Wie oft gehst du ins Kino?
How often do you go to the theater?
as a temporal preposition, “in” can be translated as “in,” where a period of time is implicit. Take note that, as a temporal preposition, “in” is dative:
In einer Stunde bin ich wieder da
I’ll be back in one hour
Was machst du im Sommer?
What do you do in the summer?
“in” is not declined (it doesn’t change), just like all the prepositions.
"im" is the contraction of (in + dem).
"ins" is the contraction of (in + das).