Densifiers tend to consist of a silicate which will chemically react with the calcium hydroxide that makes up concrete, filling the empty spaces with a chemical compound (C-S-H, or calcium-silicate-hydrates) that serves to densify the material and alleviate the problematic dusting phenomenon. This process is remarkably similar to how concrete is produced in the first place; thus, it is a good precautionary measure to take to improve the life of your concrete.
A good concrete densifier will penetrate the surface of the concrete increasing the strength and hardness by up to 50%. It is best to use a lithium silicate densifier like the Lithi-Tek 4500 since the lithium densifiers have a smaller particle size it allows them to penetrate deeper into the concrete. If you are going to be polishing the concrete you would first apply a densifier. If you are looking for waterproofing protection it is best to first apply the Lithi-Tek 4500 wait seven days and apply a topcoat such as the Siloxa-Tek 8500 or the Lithi-Tek 9500.