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You can read the documentation for is_real by evaluating x.is_real?: "Return True if this expression is known to be a real number." In particular, it is defined for expressions, which in Sage can include variables. If you evaluate parent(x), the answer will be Symbolic Ring, the same as parent(pi).

RR, on the other hand, is "An approximation to the field of real numbers using floating point numbers with any specified precision." For something to lie in RR, there must be a way to convert it to an actual floating point number, so pi in RR will return True, whereas x in RR will not:

sage: RR(pi)
3.14159265358979

and

sage: RR(x)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)

  [snip]

TypeError: Cannot evaluate symbolic expression to a numeric value.

and

sage: RR(I)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)

  [snip]

TypeError: unable to convert '1.00000000000000*I' to a real number