Among the real numbers, some are integer.
Others are rational, i.e. they are solutions of a linear equation such as
$$px=q$$ where $p,q$ are integer. The numbers not falling in this scheme are called irrational.
A rather obvious generalization of this principle are numbers that are solutions of a polynomial equation such as
$$px^3+qx^2+rx+s=0$$ where $p,q,r,s$ are integer (any other degree can do). These numbers are called algebraic, which is the converse of transcendental.
The algebraic numbers enjoy a special property: even though there is an infinity of them, they can be numbered (they are said to be countable). By contrast, the transcendental numbers cannot, there is a "larger" infinity of them.
You easily understand that all integers are rational and all rationals are algebraic.
Among the functions of the real variable, some are polynomials.
A rational fraction is the quotient of two polynomials, i.e. a function $y=\dfrac{Q(x)}{P(x)}$, that verifies an equation like
$$P(x)y=Q(x).$$
More generally, an algebraic function $y=f(x)$ is such that it can be expressed as the root of a polynomial with coefficients that are themselves polynomials in $x$:
$$P(x)y^3+Q(x)y^2+R(x)y+S(x)=0.$$
A function that is not algebraic is called transcendental.
Looking closer, one can observe that algebraic items are defined from equations that use a finite number of additions and multiplications. Transcendental items require "stronger" tools (such as an infinite number of terms).