Run this command in the same directory as you run vagrant up, your trellis directory. This command is just vagrant provision, which runs the Trellis dev.yml playbook for you, but sets the ANSIBLE_TAGS environment variable just for the duration of this command.
Ansible tags are a means to running just a portion of the playbook, e.g., just the wordpress-install role that creates the .env file. If ANSIBLE_TAGS is set, Vagrant will pass those tags to Ansible and the dev.yml playbook. It is not required to use ANSIBLE_TAGS, but it saves time when you only need to run 1-2 roles instead of the whole dev.yml playbook.