Hello,
What Bogdan says about Levels is correct, however it's link to the lock isn't quite accurate.
Drivers (or staff) with a higher level than your manager level will have red stars (as opposed to yellow).
The lock on a free agent driver indicates that they haven't been a member of your team in the past. You cannot buy a driver who hasn't been in team in the past, unless it is through the Auctions or your Headquarters.
Any free agent driver you are not eligible to purchase for this reason will show with a lock on the Contract page.
The reason for this lock being added was to stop tech savvy managers using scripts to hoover up all of the high level drivers and hoard them within their teams.
More information on this can be found on the
changelog.
I hope this helps. :)