A taxable item or situation is one that must be paid for with money to the government. When you buy something new, like a house or a car, it might be considered taxable.
When you earn some extra money from selling an old item on eBay or getting a bonus at work, it's not just yours - the government also gets a part of it because it's considered taxable income. Even if you don't think about it much, taxes are usually included in the price of things like cars and homes when you buy them, so they're already built into the cost. That's why something is considered taxable: because the money for those taxes has to be paid at some point, usually when the item changes hands or when you file your tax return each year.
Something on which tax must be paid.
Subject to taxation.
