For users running Chrome 80 and higher, Chrome is enforcing a secure-by-default cookie classification system, treating cookies that have no declared SameSite value as SameSite=Lax cookies. Only cookies set as SameSite=None; Secure will be available in third-party contexts, provided they are being accessed from secure connections. Do note that the SameSite field is not yet widely supported in older browsers, as well as Safari and Firefox.
A more comprehensive explanation of this policy explained by Rowan Merewood - Developer Advocate for Chrome @ Google - can be found here.
How to fix (or prepare) for it?
A comprehensive link with sequence diagrams can be found here. Do note that your cookies do have to be set with the correct fields so that the warnings/errors do not appear.
If you are looking for guides on how to modify your cookies based on your current stack, take a look at this link, which has examples on how to properly configure the cookies based on the stack that you are working with.
If you are wanting to test it out and are unable to see the warnings, you have to enable the experimental flags, which can be found on chrome://flags, and set all the functions named "SameSite" to "Enabled" in order to fully test the flow to see if your solution is working.