SSH (pronounced "es es aitch") literally means Secure Shell but as a user we think of it as a scheme for getting a terminal window to open on our local computer, but where the typed commands are executed on a remote computer. If you type ssh in a terminal window, you are probably asking for a connection to a remote computer.
In common usage we say ssh address. That means to open a window which shows a text terminal session on computer "address". Macintosh and Linux computers commonly use a text shell. On both, when operating from a GUI desktop, the program run to get a text shell is called Terminal or something like that. On the modern Raspberry PI installation that would be called LXTerminal. On a modern MacOSX computer that would be called Terminal. On a MSWindows machine you'd have to download and install a program, I think. Your author's preferred program for MSWindows is called PUTTY. See below for a list of links.
An SSH connection is secured. A password or 'key' exchange takes place to prove to the target computer (the Raspberry PI) that you are who you say you are. By default the target computer will ask you for a user name and a password. The syntax or configuration for calling the target computer will allow you to specify the computer prefixed by the account name and a @ sign. For instance, in PUTTY, I can set the remote computer as firstname.lastname@example.org. That tells it to log in to account pi. Now the remote computer only needs to prompt me for the password. We can eliminate that prompt as well by setting up a key exchange. This involves setting a file up on the Raspberry PI in a folder in the pi account, and also sharing a key string both with your workstation and with the Raspberry PI. Instructions for doing this on a Macintosh are below. There are instructions for doing this on MSWindows elsewhere. You are on your own with Linux. I used to have my Raspberry PI set up this way but I kept on replacing the Raspberry PI and I log into the PI from so many different computers that I just haven't set it back up again recently.