Debug Screen
Pressing F3 brings up a debugging screen which shows some useful information and allows you to change some settings.
Information
There’s a lot of information on the debug screen; here is a guide to just the useful bits.
x, y, z coordinates
x and z are your map coordinates. x is your position in the East-West direction – the number gets bigger as you go East. z is the North-South axis: numbers get bigger as you go South.
In both cases it’s easiest to look at the number in brackets: I would read the position in the above picture as 403, 1206.
y is your altitude. It measures the height above the bottom of the world, and tells you the position of both your feet and your eyes. 64 is sea-level.
f tells you which direction you are facing.
Biome
This tells you what biome you’re in.
Light Level
rl is the light level in the block where your head is.
Settings
Press F while holding down F3 to toggle the view distance to suit the performance of your machine.
More information
For a full explanation of all the advanced information and settings here, see the Minecraft Wiki.