Recently, Windows 11 was announced, and its features and requirements released. In a surprising move, Microsoft is requiring devices to have TPM chips in order to update to the newest version. A TPM is a Trusted Platform Medium chip. These are used to protect encryption keys and user credentials among other things. The goal behind requiring these chips is to help protect from ransomware and other modern attacks. Devices without a TPM can still upgrade, but they will either need workarounds or changes to the BIOS.
An article from The Verge covering this topic can be found here.
A list of compatible CPUs can be found here.