VBUS refers to the power line within a USB connector and cable that carries voltage from a host or charger to a connected device. Alongside the data lines, VBUS is one of the fundamental wires that has been present since the earliest USB specifications.
In basic USB connections, VBUS carries a standard 5 V, but with USB Power Delivery, the voltage on VBUS can be negotiated much higher, up to 20 V under standard PD or up to 48 V with Extended Power Range, to support fast charging of laptops and other demanding devices.
The behaviour of VBUS, including how much current it can safely carry, is closely tied to what a cable and its connectors are rated for, which is part of why cheap or poorly made cables can pose a safety risk under high power charging. See our Power Delivery guide for more.