Ratio is a relative measure of how many times a value fits into another and Proportion is a statement of ratio to be applied to another value.
So if you think of pieces of paper, an A5 piece of paper is roughly 148mm Wide x210mm High. (I will add I have no control of manufactured sizes and this is for illustration only)
This Gives a Height Ratio of 1.419 to the width. This can be shown by 210/148= 1.419 (rounded up)
If we take a piece of A4 paper, although the physical Dimensions are different 210mmx298mm, the ratio between the dimensions is not.
298/210 = 1.419 (rounded down)
This is then expresses as 1 : 1.419 meaning for every 1 unit of the width, we have 1.419 units for the Height.
This means that we can describe the A5 and A4 pieces of paper as being proportional in size as each other i.e. Twice as Big.
You may have heard of a term called ‘scaling up’, that is essentialy what we have done with the piece of paper and is done using proportions.
Video on Ratios
Video on Proportions