The temperature of you mash influences two things in your beer: body and ABV. In general, most homebrewing forums recommend hitting between 150-158 degrees, but even a couple degrees within that range (and beyond) can greatly influence these two parameters of your beer.
First, let's talk about body...
In general the warmer you mash, the more body you will have in the final product. This is because Beta Amylase (one of the two key enzymes in starch conversion) start to denature (stop working) more rapidly the warmer they get. Beta Amylase is responsible for producing simple sugars the most rapidly in your mash. These simple sugars are easily fermented and therefore provide no body to your final product. With less active Beta Amylase, you end up with more complex sugar molecules, that aren't easily fermented. These complex sugars are what you are after, if body is desired in your beer.
In general, follow these rules to manipulate the body of your beer.
- Mash between 149-151 for dry beers with little body.
- Mash bewteen 151-154 for medium bodied beers with some sweetness.
- Mash between 154-158 for full bodied beers with lots of sweetness.
How this affects your final gravity and ABV...
Alpha Amylase is the other enzyme that breaks down starch in your mash. It operates well within most of the typical mashing ranges (149-162), so Beta Amylase is truly the driver for the amount of fermentable sugars you will have. Like before, the warmer you mash, the less effective Beta Amylase is at breaking down complex sugars that can't be fermented. Therefore, the warmer you mash, the less alcohol you will get in your final product. Body and ABV have an inverse relationship in this way. The more body you have, the less alcohol you will have and vice versa. If you have a final gravity that seems high, it's possible you mashed too warm.