It means whenever I open a new tab of terminal, no need to reactivate my environment again. Is there any command to do that?
You can update your shell config to activate your env automatically. In bash for example you can do that via ~/.bashrc. Conda even asks as the very last step of the installation if a user wants this feature enabled.
Another approach is to use a terminal multiplexer such as GNU/Screen or tmux, one can activate conda, start tmux and have multiple tmux windows and all the other cool features.