I have a question about notation for you: What is meant by the two eighths that are set equal to each other you can see in the picture?
This from Ravel's Rhapsodie Espagnole, IV. Feria, for piano duet.
This looks like a mistake. In fact, it might be a failed attempt to correct a similar mistake in the first edition of the orchestra score.
In the orchestra score, a similar mark occurs two bars before, where the time signature changes from 3/4 to 6/8. At that position it is meaningful, to show that the 8th notes in the 6/8 are the same speed as the 8th notes in 3/4. Without the marking, an alternative interpretation might be that the dotted-quarter beats in 6/8 were the same speed as the undotted-quarter beats in 3/4.
However there is a typo in the orchestra score (see https://imslp.org/wiki/Special:ImagefromIndex/134638/torat) . In big scores the tempo markings are often repeated above the string section, not just marked above the top staff of the score. But here (rehearsal mark 19, page 69) above the top staff it says "8th-note = 8th note", and above the string section it says "quarter-note = 8th note". They can't both be right!
As Basstickler suggests, this mark is usually put where there is a time signature change, to indicate, for instance, that 4/4 time changing to 3/4 time keeps the same length for each quoted note - in my example, each bar will take three quarters of the time in 3/4 as it would have in 4/4.The other alternative is that each bar in that 3/4 takes the same time to play as it did in 4/4.
Here, the time sig. goes from 3/4 to 6/8, although I can't see it marked.I imagine that each bar previous will be the same length in time as that after, with 'quaver=quaver'. The only real change will be the feel, and emphasis, in the way the tune is played