Set an environment variable with an agent enrollment secret
The enrollment secret is a value that osquery uses to ensure a level of confidence that the host running osquery is actually a host that you would like to hear from. There are a few ways you can set the enrollment secret on the hosts which you control. You can either set the value as:

an value of an environment variable (a common name is OSQUERY_ENROLL_SECRET)
the content of a local file (a common path is /etc/osquery/enrollment_secret)
The value of the environment variable or content of the file should be a secret shared between the osqueryd client and the Fleet server. This is basically osqueryd's passphrase which it uses to authenticate with Fleet, convincing Fleet that it is actually one of your hosts. The passphrase could be whatever you'd like, but it would be prudent to have the passphrase long, complex, mixed-case, etc. When you launch the Fleet server, you should specify this same value.