Well, a thrust bearing is basically a device on the top of the tower at left that your mast sticks through to allow the mast to be anchored and still rotate. Or it can be mounted to a wall or a post as shown at right.
The thrust bearing has two main parts. The top part clamps to the mast with large set screws. The top part rides on roller bearings in a bottom housing that can be bolted to a plate on the top of the tower or on brackets fastened to the wall or a post.
The photo at bottom left shows the eight large set screws that clamp to the mast which runs through the center. The triangular plate here is designed to fit into a tower and be bolted to the three legs of the tower. So it fastens the mast securely but allows it to rotate freely and be turned by the rotator which is also mounted below on another triangular plate.
At right you can see the bottom part that houses the roller bearings. The top part turns; the bottom part remains stationery and has mounting bolts to fasten to a plate or brackets. It is not too clear that the bottom remains stationery as it looks like it is bolted to the top but it is not.
Another good thing about the thrust bearing is that it supports the weight of the mast and antenna instead allowing all that weight to bear down on the rotator. So the stress on the rotator is reduced by the action of the thrust bearing.
Most thrust bearings for ham use are found in conventional towers and thus the triangular plate arrangement. But the thrust bearing can be mounted on a couple brackets on a wall or a wood post so that the functions of the thrust bearing can be useful for push up masts.
Thrust bearings come in different sizes and prices usually well under $100. Ham Radio Outlet has them. Be sure that the one you select will accommodate the size mast you plan to use. The specs for the thrust bearing will tell you the range of sizes the bearing will handle.