Some things you can do when selecting "Custom expression":
x: the variable
x + 1, x - 1, x * 2, x / 2: addition, subtraction, multiplication, division, respectively. Respects order of operations.
sin(x), cos(x), tan(x): trig functions
asin(x) etc.: inverse trig functions
sqrt(x): square root
PI, E: constants
pow(x, 2): raise x to the second power
All of this is case-sensitive.