A densifer does just as described - it densifies the concrete and thus makes the surface harder. It is used for concrete polishing, anti-dusting, and for making the concrete surface more abrasion resistant, just to name a few uses. It does this by creating a reaction with the calcium hydroxide in the concrete to produce calcium silicate hydrate (CSH). This is the substance that gives concrete its strength. Then the CSH that is produced fills the open pores of the concrete increasing its density.
The result of this action creates a concrete surface that is much harder and does not absorb liquids as well. It's technically not a sealer, however, since liquids can still penetrate the surface if given some time. How well a densifier works depends on quality of the concrete as well. Poor concrete will not have as good a result as good concrete.
Penetrating sealers, such as the siliconate you were asking about, actually form a barrier in the concrete right at the subsurface that will not let liquids penetrate at all. Because penetrating sealers are not an actual film forming topical coating, oils can still possibly stain just the very upper areas of the concrete if left to sit. If you wipe up any stain potential liquids within a reasonable amount of time, then generally you will be fine.
