The "exp" stands for "exponential". The term "exp(x)" is the same as writing
e^{x} or e^x or "e to the x" or "e to the power of x". In this context,
"e" is a universal constant, e = 2.718281828... it goes on forever but you
don't need to know the value, your calculator probably has exp(x) or e^x as a
function (if, as I am assuming, it is a scientific calculator).

It might become obvious to you if you see the equations written properly
rather than in ascii text. Check the quick reference page,

http://www.execpc.com/~culp/rockets/qref.html

where the single stage equations are also written by way of review. It might
be easier for you to understand what is meant by exp(x) when you see it
written properly as e^{x}.

The inverse of e^{x} is ln(x), or the natural logarithm of x. So in
other words, if I take the natural logarithm of e^{x}, I get x back:
in equation form ln(e^{x}) = x, or equivalently, ln(exp(x)) = x. It
works the other way around, too, exp(ln(x)) = x.

The expression 1-exp(x) means raise the number e to the x power then subtract it from 1. So you would say it "one minus e to the x".