What does the "Object Entry" option do?
In About Land window's Options tab you'll see checkboxes for Object Entry.
This simply gives you, as a landowner, the ability to block other Residents' objects from coming onto your parcel. For example, if there are people passing by and leaving litter all over, restricting Object Entry will bounce the objects back outside your parcel, and show them a message like this:
How is this different from turning off "Create Objects"?
The Create Objects options block objects from being rezzed on a parcel.
The Object Entry options block objects being rezzed on a parcel and from entering if they're already rezzed inworld. Sometimes, there's unfortunate behavior like a griefer noticing you have Create Objects off, but a neighboring parcel has it on, so they try to annoy you by shooting projectiles from next door. Thus, Object Entry blocks them from doing that -- the two features can be used complementarily.
A situation that causes confusion which you should be aware of: On group land, a landowner might uncheck All Residents for Object Entry. Then, if a group member doesn't have the correct group active and tries to rez something, the object suddenly gets pushed out.
Tip: To set your active group, go to Edit > Groups and if desired, change it to the same group that the land is set to.
Why are there two checkboxes for "All Residents" and "Group"?
This offers you a greater level of control. Just like the checkboxes for Create Objects and Run Scripts, as a landowner, you can choose whether you want to stop everyone else from being able to push objects into your parcel, or if you want to permit this for the group the land's set to (it doesn't need to be deeded). This way, you could allow friends belonging to that group to play with objects on your parcel, while those not in the group aren't allowed to have objects enter.
Doesn't blocking "Object Entry" cause problems if you're in a vehicle?
No. If you're actively driving a vehicle, you will be able to cross through parcels with Object Entry turned off.