Peepul writes:
I think putting 'collectively agreed' in your statement makes it meaningless. Who is to do the agreeing here?
This concept goes back to the extreme basics of philosophy and the way in which we think. For example, put aside everything that you know and start from scratch:
The famous line is "Cogito ergo sum" or "I think, therefore I am." Since you are thinking you can reasonably conclude that you exist in any sense that will be meaningful to yourself. The next step is to consider what other things exist, and this leads us to our senses.
Starting from within our mind, we have no way of determining if our senses are reliable. Most people will readily admit that they can make mistakes, but the more subtle point is that *everything* could be a perfect illusion to our senses ("The Matrix" is often referenced here). There is a certain leap of faith required to conclude that anything we observe can be trusted to be real.
Once we trust our senses to a certain extent, we can determine that there are other thinking beings in the world. This lets us distinguish between consistent and inconsistent observations; for instance if you saw a fairy and 9 other people did not, you could conclude that your senses were simply being unreliable (as they commonly are). Note that this does not strictly prove that these consistent observations are objectively real, only that if they are illusions that they are shared illusions as opposed to solitary ones.
Now for the concept of objective reality; this is again something of a leap of faith. Objective reality is something that exists independent of the mind, and if you will recall up to this point your own mind is the one thing you can most reliably conclude exists. Everything beyond that is to some extent going out on a limb. The concept becomes defined in an awkward fashion; our observations indicate that things happen and exist regardless of them being observed. This again is summed up by a famous saying, "If a tree falls in a forest and no one is around to hear it, does it make a sound?" Our observations indicate that it does, but that sort of negates the whole "lack of observation" concept in the first place.
So to wrap things up we can be reasonably sure that an objective reality exists in the same sense that we can be sure anything exists, including other minds. This is to say we are not completely certain, but sure in every reasonable and useful sense. Determining this objective reality is done through weeding out individual experiences which are obviously not objective, and whatever we are left with is concluded to be objective (although it could technically be a shared illusion).