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.
tan(x): trig functions
asin(x)etc.: inverse trig functions
sqrt(x): square root
pow(x, 2): raise x to the second power
All of this is case-sensitive.