Click on the 'tutorial article' in blue in the Overview section. That is a PDF file that UPMan put together to explain trunking
'Triple Trunking' refers to the types of trunking systems the radio can handle - in this case, Motorola, M/A Com (GE) and LTR. I would suggest going down to the Missouri forum and asking about which scanner(s) would be appropriate for your area. Local scannists are always the best choice when asking about scanners for a region. You might be able to get away with a lower-cost scanner - however I believe Missouri is moving toward a digital statewide system, and that, unfortunately, is considerably more costly. There's some information on this someone has been constructing in our wiki.