Two ammonia molecules are bound to a silver ion. In the name, di means two ligands and ammine means the ligand is ammonia. A white precipitate of silver chloride dissolves in ammonia because the silver ion becomes this complex ion, which stays in solution.
The coordination number is 2 and the geometry is linear. Nitrogen, silver and nitrogen sit on one straight line, so the bond angle is 180 degrees. With only two ligands, the one arrangement that keeps them as far apart as possible is to place them on opposite sides.
The solution is colorless. Tollens reagent is an ammoniacal silver nitrate solution containing this ion; warming it with an aldehyde deposits silver on the glass and leaves a mirror. That is the silver mirror test.