What is this?
This tool helps you "preview" what a setup will look like. You can change all the variables. The blue lines represent your tires. Black is your rim, and the red is your fender. Since the fender is different for every car, you will need to enter this value based on your vehicle to get it accurate. This distance is measured from the hub to the fender. After all variables are entered will attempt to give you a visualization of poke and stretch. There's two wheels so you can compare setups.
Information on calculations:
Your tire probably has a series of numbers/letters on the side that look something like 195-45-R17. The first number is your tire width. The second is your tire profile. The tire profile is a percentage of the width. So if it's 45 and the width of the tire is 195, that means your tire's profile is 45% of 195mm or about 88mm. The last number is the diameter of the wheel in inches. The only numbers you're missing now are the rim width, offset, and spacer dimensions. If your vehicle is stock, you can leave the spacer value at 0. If your vehicle has adapters or spacers, you should know those values. The offset and wheel width are usually written inside the rim somewhere.