He means to prevent movement.
Some scopes have a "push" to lock/un-lock, some use a brake affair where you essentially expand a set of brake shoes out on the turret to prevent movement. An example for that system is the NightForce BEAST.

I've waffled on that idea personally. I have scopes that have locking turrets and scopes that don't. When I started, I thought I HAD to have them, at this point I really don't care. Never had one get "spun" unless flopping around when driving to AZ. I always check things out before shooting anyway, so my current mindset is that it doesn't matter.