When preparing a wine-press where the grapes would be placed and trodden upon, usually hot tar would be poured on the ground so that the juice would not soak into the earth and be lost. It was common practice to pour wine on the tar – either before or after it was set (Rashi suggests that it was done when the tar was still hot) – in order to help remove the bad smell and taste of the tar.
The Mishnah on today’s daf (=page) describes that if a non-Jew poured this wine it must be dried up and removed before the wine-press can be used for kosher wine. As the Gemara explains, drying up the wine should be understood to mean that it is washed off with water and detergent in order to completely remove the non-Jewish wine that may have been absorbed in the tar or mar have congealed on it. The Ra’avad explains that the purpose of washing off the tarred area was not to remove the wine, which does not really become absorbed in the tar – and if it does become absorbed, in any case it is no longer considered wine. Rather, the concern in this case is that there are spots where the tar may not have reached, where the forbidden wine may collect. Those places must be washed well before using the press for kosher wine.
The Ramban points out that even though utensils that hold wine for only a short period of time ordinarily do not need more than simple rinsing in order to be used, in the case of the wine-press, since non-Jewish wine is often left for several days in order to remove the taste of the tar – and occasionally the wine is changed two or three times – the requirement is that a more thorough cleaning take place.