An ionotropic receptor is a molecule in a cell membrane that can open or close a channel into the cell. When the ionotropic receptor is open, small particles called ions can travel into or out of the cell.
In general, ionotropic receptors are closed until a small molecule called a ligand binds to the receptor, causing it to open. When the ligand binds to the receptor, the protein that is blocking the channel changes shape to allow ions to travel into or out of the cell. The channel opens as soon as the ligand binds to the receptor and closes again once the ligand detaches.