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.
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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.
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Either search for the package name (e.g. “megaAVR”), or the board (e.g. “Uno”, “MKR1000”, or “Portenta”), by typing in the search field.
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Find the package that includes your board.
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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.
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Wait for the installation to complete.
Boards Manager in Arduino IDE 1.x. 
Boards Manager in Arduino IDE 2.
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.
