"Satisfies" is a common math term. For instance given
x + 3 = 4
the value of x which satisfies the equation is 1. To satisfy is to be a value which makes true some truth valued formula (such as an invocation of a predicate like blue(x) or equation like the above, or inequality or such).
Satisfiability comes up in logic; a "SAT" problem is, in a nutshell, the problem of finding the combination of true/false values of a bunch of Boolean variables, which make some formula true.
When the Haskeller says that "Num a" is something that is satisfied by a, it means that it's a kind of predicate which says a is a Num. That predicate is true of an a which is a Num.
Satisfiability comes up in logic; a "SAT" problem is, in a nutshell, the problem of finding the combination of true/false values of a bunch of Boolean variables, which make some formula true.
When the Haskeller says that "Num a" is something that is satisfied by a, it means that it's a kind of predicate which says a is a Num. That predicate is true of an a which is a Num.