"They have a big house."
Translation:Tha taigh mòr aca.
No, your sentence makes no grammatical sense in Gaelic (it looks like a failed attempt at saying a big house is them or something along those lines).
The Gaelic sentence means very literally a big house is at them where the aca part means at them – and that’s the typical expression in Gaelic to say that they have something.
It’s explained in the tips and notes to the lessons: first the construction is introduced in the Pets skill, then further explored in Family, and finally the form aca in the Home skill. You can see the notes in the web browser version of Duolingo (unfortunately not in the mobile app), and on the duome.eu website.