LED screen

BBC micro:bit with all 25 LEDs lit
The BBC micro:bit LED screen consists of 25 red LED lights arranged in a 5X5 grid (5 LEDs across by 5 LEDs down).

Which LED?

You use x , y coordinates to specify a particular LED in the grid; where x is the horizontal position and y is the vertical position (0, 1, 2, 3, 4). To figure out the x, y coordinates, position your micro:bit horizontally, like a credit card (see picture above).
Here are the x, y coordinates for the LEDs in the 5X5 grid:
0, 0 1, 0 2, 0 3, 0 4, 0
0, 1 1, 1 2, 1 3, 1 4, 1
0, 2 1, 2 2, 2 3, 2 4, 2
0, 3 1, 3 2, 3 3, 3 4, 3
0, 4 1, 4 2, 4 3, 4 4, 4
The x, y coordinates for the LED in the centre of the grid are 2, 2. Starting from 0, 0 count over 2 columns and then down 2 rows.

Row, column - 1

Since the row and column numbers start at 0, an easy way to figure out the x, y coordinates is to subtract 1 from the row and column number (when counting from 1). In other words, to specify the LED in the 4th column 5th row, subtract 1 from each number to get coordinates 3, 4.

Turn a LED on/off

Use plot and unplot to turn a LED on or off

Is a LED on/off?

Use the point function to find out if a LED is on or off.

Display images, strings and numbers

Instead of turning individual LEDs on or off, as above, you can display an image directly to the screen or show text/numbers on screen using the show number/show string function.

The display buffer

The BBC micro:bit runtime keeps an in-memory representation of the state of all 25 LEDS. This state is known as the "display buffer" and controls which LEDS are on and which are off. The plot/unplot/point functions access the display buffer directly. On the other hand, the functions that show an image, number or string overwrite the buffer completely. To illustrate, first try running this code sequence
basic → show string("d", 150)
led → plot(0, 0)
You will see the letter "d" displayed as well as the LED in position 0,0 lit up. Now try reversing the order of the two statements above:
led → plot(0, 0)
basic → show string("d", 150)
You will not see the LED at position 0,0 lit up because the show string function overwrites the whole display buffer.