A board core (or just board for short) is a collection of files needed to compile and upload sketches for a board. Cores are contained in packages and can be installed using the Board Manager.
Add a board with the Boards Manager
Official Arduino cores can be conveniently installed with the Boards Manager tool.
In the menu bar, select Tools > Board > Boards Manager. In IDE 2 you can also access it by clicking the button in the left-most sidebar.
Either search for the package name (e.g. “megaAVR”), or the board (e.g. “Uno”, “MKR1000”, or “Portenta”), by typing in the search field.
Find the package that includes your board.
Click Install (the latest version is selected by default).
If the package is already installed, you will instead have the option of installing a different version.
Wait for the installation to complete.
You can now select boards from the installed packages in the Tools > Board menu:
Add additional packages to the Boards Manager
See Add or remove third-party boards in Boards Manager.
Install a board manually
Cores can also be added directly to the file system. See Find sketches, libraries, board cores, and other files on your computer.