A bus represents a collection of electrical wires on a schematic sheet. The collection of wires are related in functionality, e.g. an address bus contains a collection of bus wires, say A0 to A31 for a 32 bit wide address bus. You connect several wires to a bus and the bus 'carries' the wires around the schematic grouped into a virtual single cable conduit.
Below is bus named Address which carries connections from the device on the left.