Mishnah 7 in Tractate Shabbos deals with the laws of building on Shabbos. The halachah is that one may not even put up a temporary structure, known as an 'ohel arai'. For example, laying a mat across four walls, the way one lays s'chach on top, is forbidden even though there is nothing permanent about it - and this is because of a decree, lest one come to make an 'ohel keva', a permanent structure. The question our Mishnah discusses is the law of adding on to a temporary structure on Shabbos, and the Tannaim disagree about it.
What is a 'pekak hachalon':
The Mishnah opens: "Pekak hachalon" - the window stopper. This does not mean a window one can see through, but their version of a shutter: a board that was made to fit the dimensions of the window opening, whose function was to seal the window when they wanted to close it. The question is what the law is regarding placing this stopper into the window on Shabbos.
The view of Rabbi Eliezer:
According to Rabbi Eliezer there are significant limitations on the ability to seal the window on Shabbos: "Bizman shehu kashur v'taluy - pokekin bo" - when it is tied and hanging, one may seal with it. That is, two conditions are required:
Tied - that the stopper is tied to the window.
Hanging - that it is not resting on the ground but suspended in the air.
When these two conditions are met, it is plainly evident that the stopper is designated to be put in and taken out, and the matter is so clear that placing it does not even constitute adding on to a temporary structure. Therefore one may seal the window with it.
"V'im lav - ein pokekin bo" - and if not, one may not seal with it. If one of the two conditions is missing, whether it is not tied, or it is tied but not hanging and instead rests on the ground, it is forbidden to place it in the window. The reason: Rabbi Eliezer holds that one may not add on to a temporary structure on Shabbos, and even to a temporary structure nothing may be added.
The view of the Sages:
And the Sages say: "Bein kach uvein kach pokekin bo" - either way one may seal with it. Whether it is tied or not tied, one may place the board into the window on Shabbos. For the stopper was set aside for this purpose from the outset, and it is therefore evident that this is nothing more than something temporary. The permission stands even without those conditions that would identify it more clearly as an item meant only for putting in and taking out, because the Sages hold that one may add on to a temporary structure on Shabbos. Accordingly, there is no need for it to meet the conditions of being tied and hanging from the window.
In summary: The Mishnah deals with placing the window stopper on Shabbos, and the Tannaim disagree over the fundamental law of adding on to a temporary structure. Rabbi Eliezer, who holds that one may not add on to a temporary structure, permits it only when the matter is completely evident - when the stopper is tied and hanging. The Sages, who hold that one may add on to a temporary structure on Shabbos, permit it in any case, since the very fact that the stopper is designated for this task reveals that this is nothing more than a temporary arrangement.