Hello Thom Lavarenne:
Basically, real-world filters (especially FIR filters) do not look rectangular; Instead, it looks like the figure attached (showing a simple low-pass filter). It will start rejecting signal from freq 'f1', where the 'pass band' ends (roughly; not a precise definition). The performance of the filter will keep improve until freq. 'f2', where the stop band starts (again, roughly). Transition width is the difference between these two frequencies. In case of the figure above, the cutoff frequency is 50kHz, and the transition width is (60kHz - 50kHz) = 10kHz.
If you use a FIR filter and use a small transition width, processing speed of the program will degrade, as more filter taps are needed. If you use a large transition width, the speed of the program improves (less taps are needed), but you have to deal with the lousy frequency response of the filter. There are filters which can give you a small transition width without increasing the computation complexity, but those filters often do not have desired phase responses (you usually want to have a linear phase response; non-linear phase response).
If you want to customize your filter for the best results, you can use GNU Radio Filter Design Tool. (GNU Radio Companion - Tools - Filter Design Tool).
Regards,
Kyeong Su Shin