Pressing F3 brings up a debugging screen which shows some useful information and allows you to change some settings.
There’s a lot of information on the debug screen; here is a guide to just the useful bits.
x, y, z coordinates
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.
This tells you what biome you’re in.
rl is the light level in the block where your head is.
Press F while holding down F3 to toggle the view distance to suit the performance of your machine.
For a full explanation of all the advanced information and settings here, see the Minecraft Wiki.