A ratio is the amount of one thing you have relative to the amount you have of another. If you are making pink paint from red and white paint, you would get a darker pink if your mix has a higher red to white ratio.
To get a darker pink you might use three times as much red paint as white, and you would say that the ratio of red to white paint is 3:1.
The standard way to write ratios is to use whole numbers, or integers. In that example of dark pink paint, the ratio of white to red paint is 1/3 : 1 (the amount of white paint is a third the amount of red paint. But it would be more standard to write that the ratio of white to red paint is 1:3.
If we wanted to make a lighter pink paint than this, we could use only 1.5 times the amount of red paint as there is white paint rather than 3 times as before. This is a ratio of 1.5 :1 red to white paint, but more standardly would be written as a ratio of 3:2 red to white paint - for every 3 parts (spoonfuls say) of red paint there are correspondingly 2 spoonfuls of white paint. But in the end it should all be 1 part pink paint.
The proper definition of a ratio, according to the Merriam Webster dictionary, is
the relationship that exists between the size, number, or amount of two things and that is often represented by two numbers
Basically, a ratio is comparing the amounts of two things which can be represented by two numbers, split with a colon, or a fraction.
For example, if I had 10 chocolate bars and you had 5, the ratio of chocolate bars between me and you would be 10:5 or 2:1.