That phrase has been repeatedly used[1] by Hamas and other terrorist organizations calling for the eradication of Israel and its Jews; if Palestine exists from the river to the sea, there is no room for Israel. This statement is a variation on the phrase which Islamic apocalyptic fundamentalists claim the Islamic Madhi will pronounce in the last days: "Jerusalem is Arab Muslim, and Palestine — all of it, from the river to the sea — is Arab Muslim."
The Anti-Defamation League and other prominent Jewish organizations, both left and right leaning, condemned the speech. ADL vice president said[2],
"Those calling for ‘from the river to the sea’ are calling for an end to the State of Israel...It is a shame that once again, this annual event at the United Nations does not promote constructive pathways to ‘Palestinian solidarity’ and a future of peace, but instead divisive and destructive action against Israel."
Hill later claimed[3] he was not calling for the destruction of Israel. But other snippets from his speech[0] showed he in fact was arguing that the UN should permit violence against Israelis, and that peaceful protest was not the only way forward for Arabs living in and around Israel.
[0]: https://twitter.com/AviMayer/status/1067985406388510727
[1]: https://en.wikipedia.org/wiki/Palestinian_nationalism#From_t...
[2]: https://jewishjournal.com/news/nation/242732/cnn-commentator...
[2]: https://twitter.com/marclamonthill/status/106823807625274572...