A printer pulls paper from the tray, bends it around rollers, and feeds it past a hot fuser. Any point where the paper slips, sticks or creases becomes a jam. Understanding the path makes it obvious where to look.
Paper quality is the most common cause. Paper that has absorbed humidity swells and sticks together, so the printer picks up several sheets at once. Reams that have been open for months are far more likely to jam than fresh paper.
Overfilling the tray also causes multi-feeds. Trays have fill lines for a reason; too much paper lifts the pick-up mechanism above the correct height and lets it grab extra sheets.
Worn rollers lose their grip and either fail to pick up paper or let it slip sideways. If jams happen with good paper and a properly filled tray, the rollers are the next suspect.