Saddle Bags nowadays are sleek aero shaped seat packs that attach under your saddle and are designed to carry essentials such as trail tools, a small repair kit, spare tube, energy bar, or wallet.
Essential items to pack in your under seat bag:
Some feather edge patches, rubber solution, a pair of plastic tyre levers (strong and not too bendy), a piece of fine emery paper, a small adjustable spanner if using wheels with hex nuts, an allen key if using allen bolt fitted wheels