There are plenty of articles on this, so make sure to check them out to get a thorough understanding of it (read, brew, read, brew, ad nauseum), but basically the vast majority of your grains contain enzymes that are active within a specific temperature range (typically around 153F mash temperature for a single-infusion mash). Just like humans break down in temperatures that we consider extremes, so do these enzymes denature at higher temperatures. These enzymes are needed to convert the starches into fermentable sugars (sucrose) and what eventually becomes your wort that will be eaten by the yeast to make your beer. You're probably more productive in a room that's not really, really cold or hot, so think about those poor enzymes next time you set your strike temp too high. Also, you're more likely to pull off bitter, leathery tannins from the husks of the grains with higher temperatures.
But like I said, plenty of information in past threads. Read up and get brewing (taking lots of notes and reiterating them into a way that makes sense to you is helpful, too.