Jupyter Lab is an IDE where you can open notebooks, files, terminals, etc all in one interface. Additionally, you can easily adjust the layout of open files (want notebooks side by side? Click one notebook's tab and drag it to one side of the screen. Want a terminal on the bottom of the screen? Open a terminal and drag its tab to the bottom of the screen. Want 3+ notebooks side by side? Click and drag. Etc).
I basically only use the Jupyter Lab interface when I'm working with notebooks (sorry about using the term notebooks so much, it's a synonym for a .ipynb file as well as the name of a server mode Jupyter offers).
I still dont quite get that is diff btw jupyter nb vs jupyterlab.
My understanding is notebook for single user locally, while jupyterlab is multi-user running on server, something like that
Jupyter Lab lets you have multiple files/directories/terminal/csv files/json files/html pages/etc open at once in the same browser window.