Essentially, the UK recognized Egypt's complete dominion over the entirety of the Nile River in exchange for their own continued control over the Suez Canal.
For nearly all of human history right up until the 20th century, the Nile River Valley through Egypt was the most densely populated place in the world.
To the north is the Mediterranean Sea and then to the South the Nile River Valley Narrows and curves for more than a thousand kilometers through the desert.