A ratio compares two quantities by division, and a proportion is simply a statement that two separate ratios are actually equal to each other. That equality is exactly what lets you scale a recipe up for a bigger dinner party, convert a distance measured on a map into a real-world distance, or resize an image without distorting it, in every case, both individual numbers involved genuinely change, while the underlying relationship between them stays exactly the same throughout.
A proportion states that two ratios hold the same underlying relationship
Writing a proportion as two equal fractions, one known ratio set equal to a second ratio containing an unknown quantity, gives a direct, mechanical way to solve for that unknown: cross-multiplying the two sides turns the proportion into a simple equation, letting you solve for whatever specific number you don't yet know once the other three are given.
The same underlying idea shows up across genuinely different everyday tasks
Scaling a recipe up or down keeps every ingredient's ratio to every other ingredient fixed, even though every individual quantity in the recipe changes. A map's scale is itself a fixed ratio between map distance and real distance, letting you convert any measured map distance into an actual real-world one using that same constant relationship. Resizing an image while keeping its aspect ratio fixed is the identical idea applied to width and height, both dimensions change, but their ratio to each other stays exactly constant, which is exactly what keeps the resized image from looking stretched or squashed.
What we're still unsure about
That a proportion states two ratios are equal, and that cross-multiplication provides a mechanical way to solve for an unknown quantity within one, is well established, uncontroversial arithmetic. What's genuinely still a live, actively studied question in mathematics education is that proportional reasoning is deceptively easy to state as a rule but genuinely difficult for many learners to actually apply correctly to a real word problem, students who can solve an abstract proportion equation fluently still routinely struggle to correctly set up the right proportion from a described real-world situation in the first place, and exactly why that specific gap persists remains an active research question rather than a fully solved teaching problem.
This sits inside Ratios & Proportions, one of seven topics in Arithmetic, one of seven domains in Mathematics, one of seventeen subjects the app can quiz you on.