These terms are used to relate two numbers to each other.
"Proportional" means that as one number increases, the other number increases as well. The weight of your aircraft is proportional to how many pork pies your passengers had for lunch.
"Directly proportional" means that as one number doubles, the other number doubles. The distance you travel is directly proportional to how long you fly for (assuming your speed is constant).
"Inversly proportional" means that as one number increases, the other reduces.
If you have specific examples, post them here and I'll try to explain how the terms to be interpreted in your examples. There is a bit of flexibility - for example, "proportional" can often be used to mean "directly proportional", depending on the context.
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