What does "Parcel is full" mean?

I was wandering around and gathered some free objects. I tried to place them on the land some friends own (Slacker Planetarium) but I get a "Parcel is full" message. I've placed other objects and built objects from scratch, so I know it isn't a permissions issue. What's that mean? What can I do about it?

Each parcel of land can only hold so many prims. You went over the limit. Not much you can do except pick up something else before placing the new things, use lower prim items, or buy larger land.

Is there some way to view the number of prims already on a parcel? Also I deleted an object but still can't place another.

The way to view the amount of prims on a parcel is by right-clicking on the parcel, choose "about land" and go to the third tab from the left. (Sorry I can't go in game to check the title).
There you'll see all the prim info you need.

OK, I checked the land -- there are only 108 out of 117 prims used, but I still can't place a parcel. Is this another glitch? I am a member of the group that owns the land. I have tried placing multiple objects from multiple other sources and none will place.

I was able to place wearable/usable objects (Coke can, sandwich, etc.). They raise the prim count and lower the count again when deleted. But regular, non-wearable objects won't place.

objects "weigh" different prim weights a coke can is 1 prim (a small cylinder with a texture). While a freebie house will be can be dozens of prims.

You only have 9 prims left in that parcel. You will not be allowed to rez any object that "weighs" more than 9 prims.