A section is a continuous part of a sector or in some case of several nearby continuous parts. For sectors with far away parts multiple sections will be created, especially when they lie in disjoint parts of the map.
This is mainly supposed to cut down on time for linking dynamic lights. Since they need to traverse subsectors to find all touching sidedefs a more coarse data structure that only contains the info needed for this is more suitable. In particular, this does not contain any intra-sector lines, i.e. those with both sides in the same sector.
