Heathrow (London Airport) is one of the busiest in the world. It takes flights from everywhere, and is a big hub for transfers. The airport is to the west on London, roughly 17 miles out.
We’re going to assume you are arriving here and want to get to central London (not really “downtown”) to a hotel or residence perhaps. If you are here for work, you may already have a car booked and this does not apply.