Ask a linux Command, sudoadd-apt-repositoryppa: ailurus

Ask a linux Command, sudoadd-apt-repositoryppa: ailurus -- general Linux technology-Linux technology and application information. The following is a detailed description. I installed ubuntu today. I want to install ailurus, But I can use sudo apt-get ingstall ailurus in the command line to show that there is no software, and then I will search for it online.
Someone gave it
Sudo add-apt-repository ppa: ailurus
Sudo apt-get update
Sudo apt-get install ailurus

Then I can install it as I did,

What does sudo add-apt-repository ppa: ailurus mean?

After this sentence is completed, what does sudo apt-get update mean?