When there is a single switch in a VLAN segment, no tagging is required, since the switch is aware of the ports connected based on the CAM table. Things become a little complicated as we introduce more switches in the existing topology and the packets needs to commute over one or more aggregated ports, known as trunks:

A tag is a 16-bit field that is inserted into an Ethernet frame. When a switchport is encapsulated with an ISL or tagged with a 802.1Q protocol, the mechanism adds the VLAN identification number or VLAN ID.