Palestine is located between the Jordan River and Mediterranean Sea.

From the jordan to the mediterranean. It's really just like what it sounds like - all Palestinians will be free from Israeli oppression within the history borders of Palestine. There's a lot tied up in that - people returning to their homes and reclaiming stolen lands, for instance, but the basic message is just what it sounds like.

The way I see it, the political project which forms the foundation of the state of Israel is that it must be a jewish state. No zionist in the United States or Europe even questions that, they simply ask wether or not a jewish state has the right to exist - with no care given to the actual population of the region. Given the demographics of the country, Israel's objectives can only be achieved, and maintained, via a mixture of apartheid and ethnic cleansing. To say that Palestine must be freed from the River to the Sea is a recognition of how the Two State ideal is only a short term solution to criticism of israeli apartheid, allowing it more time to accomplish its goals. There is just no permutation of Israel which will allow Palestine to exist in any way, shape or form.

For as long as Israel exists, Gaza and the remnants of the West Bank will remain an archipelago of concentration camps where the state of Israel controls water, electricity, property rights, freedom of movement and information, and so on, just so that it can all be systematically denied to the palestinians. Wether in the future there is such a thing as 'Israel-Palestine', there is no future in which either Israel accomplishes it's goals of ethnic cleansing, or Israel such as it exists is abolished.

I recall seeing someone mention a version of the phrase appears in some of the founding documents of the ethnostate.

Here's what I found:

The context and the intent is key. The founding charter of Benjamin Netanyahu’s Likud party trolls: “Between the sea and the Jordan there will only be Israeli sovereignty.” [The 1977 election manifesto of the right-wing Israeli Likud party]

According to American historian Robin D. G. Kelley, the "odious" phrase "began as a Zionist slogan signifying the boundaries of Eretz Israel." Zionism is the movement to support the establishment of a homeland for Jewish people in Israel.

Palestine should be free in its borders (either 1967 or as single multi-ethnic/federation state) and not be actively colonized

A good brief overview of the history is at decolonizepalestine.com and a comrade did a write up https://hexbear.net/post/960298

it means "our opening offer is the 1967/8 borders"

If videos are your thing:

What ‘From the River to the Sea’ Really Means & Why Israel Can’t Win, w/ Vijay Prashad