im meaning and definition in german

The contraction im is obligatory when the definite article has no deixis whatsoever. In this case it is ungrammatical to use in dem as separate words:
Wir sitzen im Garten.
Er arbeitet im technischen Bereich.
On the other hand, im is usually not applicable when the definite article has an indicative function. This includes contexts in which English would use a demonstrative pronoun (“this” or “that”) but also some others. Particularly, in dem is used before a defining relative clause.
In dem Haus haben wir mal gewohnt.
Mein Großvater starb in dem Haus, wo ich geboren wurde.