There's a pretty good Wikipedia article on it

As mentioned, it's the Jordan River to the Mediterranean. It's been used by Palestians since at least the mid 60s in a number of different chants, e.g. "from the river to the sea Palestine will be free/Arab/Islamic" (technically, the latter two are "from the water to the water" because otherwise the Arabic doesn't rhyme).

Hamas's charter says

Hamas rejects any alternative to the full and complete liberation of Palestine, from the river to the sea

While Netanyahu's far-right Likud party's 1977 manifesto says

between the Sea and the Jordan there will only be Israeli sovereignty

It's historically been somewhat controversial, with Zionists typically saying that it calls for the destruction of the state of Israel and/or the expulsion of Jews from the area. CNN fired a political commentator for saying it ~5 years back, and it's regulated as hate speech in some places in Europe. Most pro Palestinian activists think that's ridiculous, but it's worth being aware of.