。
I'm looking for a present for my mother.
Literal
Mother [gen-の] present [obj-を] am-looking-for [explanatory-んです].
母のプレゼント is ambiguous on paper — 'mother's present' or 'a present for mother' — but context (looking for one) forces the latter reading. The のです/んです ending marks the sentence as explanatory: this isn't raw information, it's context the speaker is offering (to a shop clerk, perhaps). んです is the contracted, more conversational form of のです.