This is a tool which shows the projected growth of your investments in the future, as well as the milestones you will hit along the way.
Investing is a slow process, and the milesones and reason to celebrate can feel few and far between. This tool aims to give you multiple milestones to look forward and celebrate every year, using various different metrics like your safe withdrawal rate, monthly growth from interest, etc.
Not at all. Investing is always risky, and you're not guaranteed anything. The tool assumes, by default, a 7% annual growth after adjusting for inflation, but this is just based off historical data and cannot be guaranteed.
Additionally, the milestones are generated from a purely theoretical/mathematical point of view. When the tool says you are earning "$100 monthly interest", this simply means that, at your current net worth and monthly contributions, and if we assume an annual growth of 7% divided into 12 months, that comes out to be a monthly interest of $100 for that month.
Different countries have different tax laws, and different types of accounts are subject to different levels of taxation.
Some parts of your net worth might be in home equity, some in a tax-advantaged account you can only access later, some might be in an after-tax brokerage account.