If you want a more in depth explanation... here it is.
The "bit" number represents the "largest" number your CPU can process natively (i.e., without supporting code). 32 bit CPUs can hold something like 4 billion, 64 bit CPUs can do like... 18446744073709551615. omputers think in base 2, so 32 bit computers can do 2^32 - 1, 64 bit computers can do 2^64-1.
Most computers now are 32bit, owing to the fact that 64 bit CPUs only hit the market relatively recently (last few years) and that migration isn't the simplest.
The good thing however is that chances are if you don't know what 64 bit and 32 bit is... you probably can get the 32 bit version, no questions asked