Translating, with some minimal explanation:
name :: constraints_for_type => type_of_name
says "Thing named 'name' has type 'type_of_name' with constraints 'constraints_for_type'"
So,
minBound :: Bounded a => a
maxBound :: Bounded a => a
"minBound has type 'a' for any 'a', so long as 'a' is an instance of Bounded"
These two are more values than functions, but the polymorphism happens the same way. If you use them where some particular bounded type is expected, that's the type they evaluate to. If you use them where an unbounded type is expected, you'll get a compile error.
----
toEnum :: Enum a => Int -> a
"toEnum has type 'Int -> a' (that is, a function from 'Int' to 'a') for any type 'a', provided that 'a' is an instance of Enum"
i.e., Convert an int to the Enum type that the context is asking for.
----
fromIntegral :: (Integral a, Num b) => a -> b
"fromIntegral has type 'a -> b' (that is, a function from 'a' to 'b') for any types 'a' and 'b' where 'a' is an integral type and 'b' is a numeric type"
The tuple syntax just means that all these constraints need to apply. I'm not actually certain why the syntax requires it.
----
read :: Read a => String -> a
"read has type 'String -> a' for any type 'a', provided that 'a' is an instance of Read"
----
mempty :: Monoid m => m
"mempty has type 'm' for any type 'm' (different letter is just stylistic - m for monoid), provided that 'm' is an instance of Monoid"
This is particularly interesting when you start doing polymorphic things with fold and friends.
----
return :: Monad m => a -> m a
"return has type 'a -> m a', so long as 'm' is a Monad"
Here we see a "higher-kinded type" - m is a function at the type level that takes a type argument and produces another type, like a C++ template.
e.g. List parameterized by Integer gives us a List of Integers (List is spelled [] in Haskell)
----
mconcat :: Monoid m => [m] -> m
"mconcat is a function from any 'list of m' to a single 'm', provided 'm' is an instance of Monoid"
mconcat = foldr mappend mempty
"we define mconcat to be the right fold of mappend over the list, using mempty as our initial value"