There are several types of gated channels in cell membranes. The term "gated" refers to and opening or closing of the channel. Some are opened by chemical interactions, some by electrical currents or other means. These gated channels allow ions and other materials to flow in and out of the cell when they are in the open position and prevent this flow when in the close position.
Here is an example of a gated ion channel. In this case, there is a high concentration of sodium (NA+) ions outside of the cell. They cannot enter because they are too large to pass through the membrane by simple diffusion and the gated channel is closed. When the agonist is removed, the gated channel opens allowing the sodium to flow through the channel. When the agonist is put back into place the gated channel closes stopping the flow of sodium.