See
http://mathworld.wolfram.com/BinomialCoefficient.html, but don't try to read past equation number (1) if you have a weak stomach [img]/images/graemlins/wink.gif[/img]
What it means on your calculator depends on what kind of calculator you have. Some calculators have an nCr function, but if yours doesn't, you can calculate binomial coefficients by performing the necessary multiplications and division. For example, 44 choose 5 is (44*43*42*41*40)/(5*4*3*2*1).
Hope this helps,
Carsten.