In 802.1Q trunking one vlan is left untagged. This is called the native VLAN. This command tells the switch to not tag frames that are on VLAN 101 on this port and that untagged frames received on this port are for VLAN 101.
It is best practice to set the native VLAN to an unused and unrouteable VLAN as many of the VLAN hopping attacks end up on the native VLAN.
Robert
