SSH is used for connecting, controlling and executing commands on a computer. This can be really useful if you don’t want the faff of having to login to your box physically, or maybe you don’t have access to it.
First you need to install the packages:
sudo apt-get install ssh
Next you will (probably) want to start the client at boot:
sudo update-rc.d ssh defaults
Then, when you reboot (sudo reboot), it will start the client automatically.