In a small courtyard of less than four amot square, the Mishna on our daf teaches that pouring water on the ground is a problem, because it will spill out into the public domain. Even though the person standing in the courtyard is not spilling directly into the reshut ha-rabim, still it is the strength of his pouring that causes the water to move from one domain to another, so he will be held liable – at least on a Rabbinic level.
With regard to a courtyard that is less than four cubits by four cubits in area, one may not pour waste water into it on Shabbat, unless a pit [ukah] was fashioned to receive the water, and the pit holds two se’a in volume from its edge below.
This halakha applies whether the pit was fashioned outside the courtyard or whether it was dug inside the courtyard itself. The only difference is as follows: If the pit was dug outside in the adjoining public domain, it is necessary to arch over [likmor] it, so that the water will not flow into the public domain. If it was dug inside the courtyard, it is not necessary to arch over it.
It is clear that an ukah is a pit for collecting water. The Aruk suggests that it is a pit with a particular shape, specifically dug in a circular fashion.
Rashi understands that likmor means to cover the ukah with boards. According to this opinion, such an arrangement makes the pit into a makom patur – a “free space” – into which the water can be poured without concern that one is transferring from one domain to another.
According to the Meiri, this pit needs to be covered because, if it is too large or not deep enough, it will have the status of a karmelit or a reshut ha-rabim (public domain). Once it is covered it will be considered a pit in a reshut ha-yahid (private domain) or a makom patur, so that pouring water there will not be a problem.
Maimonides says that the reason for covering the pit is marit ayin – for appearance sake – so that it does not appear to be part of the public domain.
The Darkei Moshe says that we are concerned about having an open pit in the public domain, which is why we need to cover it. (For more on the issue of domains, read the Introduction to Eiruvin.)