Recently I have been playing around in terminal and typed the
who command. As I see my username and date, I don't know what
tty2 on Ubuntu 17.10 and newer) or what this
pts/0 is, can somebody explain this to me?
The name of TTY actually comes from the old days of computers: then computers even had teletypewriters as terminal, so you could see the output of programs printed (tty = TeleTYpe/TeleTYpewrite).
TTY usually refers for "physical" terminals, like more attached terminals (even like a teletypewriter) to a single computer, or in our days: more separated text consoles, you can switch between them Ctrl + Alt + F1-F7 (or more, if you have configured more; F3-F6 on Ubuntu 17.10 and newer).
In case of Ubuntu, tty7 is usually used by Xorg/Wayland display server, do provide your graphical environment (to be more exact, it provides a "windowing system" only, and things like the gnome - as a desktop environment solution - runs "top of it").
By contrast, "pts" is (one half of) a notion of "pseudo terminal" which implements terminal functionality without the context of a "real" physical terminal, for example if you open gnome-terminal, for the shell (which is usually the bash on Ubuntu systems) to be able to run, it needs terminal capabilities.
This is also the case if you use "ssh" to log in a remote machine, so in nutshell: whenever something needs terminal/tty like functionality without having a 'real' tty.
tty7 is the physical terminal from which you logged in. In many cases, tty7 is where your window manager runs and from which you login.
As can be seen from the listing below, I logged in at 9:33 on May 24 from tty7 (via Gnome window manager login dialog) and then opened 4 pseudo terminals.
[iluvblender@myhome ~]$ last | tac | egrep "May\s*24" sgoda tty7 :0 Wed May 24 09:33 still logged in sgoda pts/0 :0 Wed May 24 09:33 still logged in sgoda pts/2 :0 Wed May 24 09:36 still logged in sgoda pts/4 :0 Wed May 24 09:40 still logged in sgoda pts/5 :0 Wed May 24 09:42 still logged in