You are the little red dot. That is, you are in a spaceship in orbit around the Earth (the big blue dot). Your goal is to make a TLI burn (that's "trans-lunar insertion") and then an LOI burn (that's "lunar orbit insertion") and get into orbit around the Moon (the middle-sized grey dot). A green light on the moon will signal when you have been in a close enough orbit for long enough. Then, of course, you have to get back to Earth....
Naturally, no-one in this particular space program thought to include a computer on board the ship, so you have to fly by the seat of your pants. The yellow bar indicates how much fuel you have left. Your only control is the grey disc in the lower right corner. By holding down the mouse button with the cursor in the disc, a red line will appear indicating the direction and rate at which you are burning fuel. You can drag it around inside the disc, but not outside. Your acceleration is proportional to the rate of burn divided by your mass. When full, your ship is 85% fuel and 15% hardware. That's it!
Well, it's really not so easy. To give you a fair chance, you can fly off the screen a little ways. But if you zing out into space or crash into either of the planets then the game will pause for a second and reset you into Earth orbit again. Best of all, if you park in a close enough Earth orbit for long enough, a tanker will refuel your ship and you'll be good as new.