This is the command you use to update your system, fetch and install software. Some people use apt-get, others plain apt and the two appear interchangeable but they are NOT the same. As it’s making a change to the system, you almost always have to run it via sudo.
They are different?
Well yes. Try these.
apt --help apt-get --help
Those give different help messages. And as for these:
apt check apt-get check
It’s curious that apt-get check works, but apt check gives an invalid operation! I’m not sure why they are so similar yet subtly different. If anyone knows, drop me a line.
Having created this post, I subsequently did find out the differences- explained on this page. The simplified version is the apt is a simpler subset and also shows a progress bar when you do sudo apt upgrade. Try sudo apt-get upgrade next time to see it without the progress bar!