I have an equation for predicting the altitude of a model rocket under specific impulse. I am struggling to understand some parts of it. Here is the one equation for a variable that I need, an the variable definitions:

v = q*[1-exp(-x*t)] / [1+exp(-x*t)]

v = velocity;

q = [sqrt(T-M*g)/k]

x = 2*k*q/M;

M = mass;

T = thrust;

k = 0.5*0.75*rho*Area;

rho = 1.2kg/m^3;

t = burn time,

g = 9.8m/sec/sec (gravitational constant).

So I think I got them all. My question is the [1-exp(-x*t)] / [1+exp(-x*t)] portion,

I assume -x is x * -1; t is self explanatory.

what is exp in this case? I've never seen this before, and can't find anything that makes sense on the web. A web link and explanation would be very helpful... thank you!