Transcendental numbers are basically NON-Algebraic numbers
And to answer your your next question, "what are Algebraic numbers?"
Algebraic numbers are basically solutions of polynomials equations
degree n>0 with integral coeficients.
So algebraic numbers include all integers because of the equation
x-k=0.
Also includes all rational numbers because of ax-b=0 gives x=b/a.
And also all numbers with surds in, such as cube root of (1+rt2)
pi and e are excellent examples of transcendentals, since they
will never be solutions of a polynomial(n>0) with integral coeffs.
A few more things to say:
Algebraic numbers are countable, so have 0 measure,and take up 0%
or the real line.
Equivalently the set of transcendental numbers are uncountable, and
take up 100% of the real line.
Ironically then that you can say that 100% of the reals cannot be
written down!!!!!!