as everyone say's "it gives mass to things", but that is pretty vague, so let me explain it this way, there are 2 key things to remember here, the "Higgs field" and the "Higgs particle". it would be easiest to explain the Higgs field first. Think of it like air. when you are at "rest" you don't notice it much. but the faster you go threw it (or it goes around you), the more "resistance" you "feel". well that that is how the "higgs field" interacts with matter. the more "resistance" the matter "feels" the more mass it has. Now for the "higgs particle". Now we know that "air" is actually made of of trillions upon trillions of tiny "molecules" and it is the interaction (squeezing, moving, etc) of those molecules against what ever is moving threw them that offers the resistance. It is basically the same thing with the "Higgs particle", however.... to my understanding, the concept is almost exactly reverse, instead of pushing away, as it would be in wind resistance, it is actually attraction, as in how much of the higgs field the matter attracts, the more it attract the more mass it has, but the wind resistance reference helps ppl understand how it works.