Think of equivalent ratios as equivalent fractions, or fractions that are equal to each other.
An example of an equivalent ratio would be
1:2(or 1/2) = 2:4(or 2/4).
These two ratios are exactly the same, though they may not look anywhere near the same.
One way we could determine if two ratios are equivalent or similar, is by dividing the bigger side by the smaller side ( in our example, that would be 2/1 = 2), then divide the other side of the ratio by the same side ( 4/2 = 2 )
Since they are both equal to 2, we can say that they are the same, or equivalent.
I know this is a lot of information, but I hope this helped!