When two switches are connected together, there must be a mechanism to identify the VLAN a frame belongs to. We aren't talking about the physical layer but about the data link layer. When two switches are connected together, each one needs to know to which VLAN the traffic is destined for. This is where VLAN tagging comes in; when a frame moves over a switch-to-switch link, the source switch tags the frame with the VLAN ID, and this switch-to-switch link is known as a trunk.

Following is a screenshot of an inbound and an outbound PDU, captured in simulation mode, when a PC in VLAN 10 pinged a PC in VLAN 30:

Notice the TCI (Tag Control Information) field that contains a hexadecimal value; it denotes the VLAN ID. So, the inbound PDU has 0xa, which is a VLAN 10 source, and the outbound PDU has 0x1e, which is a VLAN 30 destination.