A proportion is a statement that two ratios are equal. A ratio is a comparison of two different quantities. "Miles per hour" is a ratio. "Dollars per pound" is a ratio. A proportion is that two of these things are equal.
For example, paying $2/pound for apples is the same as paying $6 for 3 pounds:
That's a portion.
When working with proportions in word problems, you start by setting up one ratio/fraction where you know all the information. You then set up the second one that's missing a piece of information, but you make sure to set it up using the same structure.
If your first ratio is "miles over hours," then the next ratio has to have the same organization.
If your first ratio is "rise over run," then the next ratio has to have the same organization.