The mash temp is one of the variables that will affect the relative fermentability of the resulting wort.
The two enzymes responsible for breaking down starch into sugar are alpha and beta amylase. Alpha amylase works best at temps between ~150-160 (ish) and creates a minimally fermentable wort. Beta amylase works best at temps in between 140-155 (ish) and creates a more fermentable wort.
so by mashing at 148 you maximize the beta amylase activity and therefore, in theory maximize the fermentability of the wort. a mash temp of say 154ish should produce a moderatly fermentable wort and a really high mash temp like 162 creates a minimally fermentable wort.
Less fermentable worts will produce a thicker mouthfeel and body with more residual sugars and more fermentable worts produce a more 'digestable' beer with a lower final gravity, and a lighter mouthfeel with less body.
generally speaking I mash at 148 for big belgians, big barley wines that I want to finish pretty dry, and the like. I mash at 155ish for everything of moderate gravity that shouldn't be bone dry and 162 for small beers that I want to leave with a relatively large amount of body.