Answer Description

VLAN hopping is an attack method where a host assigned to a VLAN is able to gain access to traffic on other VLANs which the host hasn’t been assigned access. The 2 primary method of this are switch spoofing and double tagging

Wikipedia

VLAN hopping is a computer security exploit, a method of attacking networked resources on a virtual LAN (VLAN). The basic concept behind all VLAN hopping attacks is for an attacking host on a VLAN to gain access to traffic on other VLANs that would normally not be accessible. There are two primary methods of VLAN hopping: switch spoofing and double tagging. Both attack vectors can be mitigated with proper switch port configuration.