Every process on a Linux machine runs as some user. Every file belongs to a user. That's how the system decides who can read what.
You already met whoami. Its sibling id gives the fuller picture:
whoami # just the username
id # username, user ID, primary group, secondary groups
A typical id output looks like uid=0(root) gid=0(root) groups=0(root).
Each user has one line in /etc/passwd, colon-separated:
username:x:UID:GID:full name:home directory:login shell
root:x:0:0:root:/root:/bin/bash
ubuntu:x:1000:1000:Ubuntu:/home/ubuntu:/bin/bash
The x is a leftover from the days when the password hash lived
there. Now hashes are in /etc/shadow, which only root can read.
Regular users own their own home folder under /home/<name> and can
run everyday commands. root is the superuser: UID 0, unrestricted,
owns most of the system, and can undo anything.
On production servers you almost never log in directly as root. You
log in as yourself and use sudo for the moments when root
permissions are actually needed.
sudo apt update # run this one command as root
sudo -i # start an interactive root shell
On this lab machine you're already root, so sudo is a no-op here.
The habit still matters everywhere else.
Groups let several users share access to a resource. Every user has
one primary group (the GID in /etc/passwd) and can be a member
of any number of secondary groups, listed in /etc/group. When
you look at file ownership in the next node, you'll see both a user
and a group next to each file.