You may wish to have an SSH server on your Ubuntu computer to give you the possibility of connecting to it from another computer with the SSH command. An SSH server can also allow you to transfer files securely from your computer to another. Ubuntu doesn't come with an SSH server installed by default, but it's easy to install one.
OpenSSH is a very popular SSH server that you can install on Ubuntu.
All you have to do is open up a terminal and install the OpenSSH server with the following command:
sudo apt install openssh-server
After the installation, the OpenSSH server will run automatically. You can then connect to your Ubuntu computer from another computer with SSH.