。
We had a meal at our uncle's place.
Literal
We [topic-は] uncle [of-の] house [at-で] meal did.
で marks the location of an action: 食事する ('to have a meal') is something the speaker does at uncle's place, not a state of being there. This is the core split learners drill for months — に for where something exists, で for where an action happens. Eating, working, studying, and other deliberate activities all take で.