A section is the Pullman invention that was the standard U.S. sleeping car arrangement before WWII -- the sleeper bunks with curtains that you see in old movies. Each section has two facing bench seats by day. At night, the two seats become the base of a lower berth, and an upper berth folds down from above; both have heavy curtains that give you a measure of privacy from people passing in the aisle.
VIA sells the upper and lower berths separately, or you can book two together as a section if you have a party of two. The lower has a window; the upper does not. Consequently the upper is the lowest priced sleeper space, while the lower costs a bit more but is still less than a roomette. The Manor sleepers that are commonly used on the Canadian each have three sections, four roomettes and six bedrooms (one of which is actually what used to be known as a "compartment," with a bit more floor space than the other bedrooms).
I have traveled many happy miles in lower berths on VIA. The price is considerably less than a roomette, though it still includes meals and the other amenities of VIA's "sleeper plus" class, and the bed is actually wider. I have never slept better on a train than in those beds. But you do share a bathroom with the other section passengers, and your space by day is not private. I have not found that I'm terribly concerned about the latter point on the Canadian, as I wind up spending a lot of the day in the domes, diner or lounge areas anyway.