One of the 39 melachot that a person is not allowed to do on Shabbat is mefarek, which is extracting liquid from a fruit. This would apply to squeezing fruits or other types of produce in order to juice them. To make sure this does not happen on Shabbat, they said that if juice comes out of a fruit on its own, without a person squeezing it, it is also prohibited to use that juice on Shabbat.
There is a machloket (disagreement) between Rabbi Yishmael and Rabbi Akiva about this gezerah (decree). There were three things—three different types of produce—that Rabbi Akiva did not agree with in Rabbi Yishmael’s ruling: cloves of garlic, unripe grapes, or unripe grain chopped up during the daytime, that is, before Shabbat started, so that the juices would then flow out on their own. They would put weights on top of the produce, and the produce would ooze out all different types of liquids.
According to Rabbi Yishmael, you are allowed to let the juices continue to flow after Shabbat starts, and you are allowed to use that juice. You can keep the weights on after Shabbat has started, because once the produce is already chopped up, the juices are going to flow out on their own without any squeezing whatsoever. If a person happened to squeeze the produce, he would not be violating the de'oraita (on a Torah level) of mefarek on Shabbat, because he is not causing any more juice to come out. It's coming out anyway. So, according to Rabbi Yishmael, since there is no problem biblically, there cannot possibly be an issue on a Torah level. He says there is no problem derabbanan (rabbinically) either; the whole problem is that you made juice. But if there is no issue of juicing on Shabbat because the juice comes out on its own, Rabbi Yishmael says, therefore, there is no need to be careful, and there is no gezerat derabbanan (rabbinic decree) either.
Rabbi Akiva says, “No, you are not allowed to let the juices come out on their own on Shabbat. You cannot leave the weights on and allow the juices to come out on their own, because, according to him, the gezerat derabbanan applies in all cases, even when the juices would come out on Shabbat in a permissible way, when you cannot actually squeeze the produce and doing so would not violate the prohibition. No matter what, no distinction is made: all juices that come out on Shabbat are prohibited.” Shabbat Shalom.