Secret Observers is a tutorial map which gives a good introduction to the new observer and piston blocks. It’s a short map but nonetheless useful if you don’t completely understand the usage of the blocks yet. Just make sure you’ve got the latest 0.15 version of Minecraft Pocket Edition before getting the map.
Basically the observer block will detect any block changes in its facing direction and then send a redstone signal if something happens. Like in this case a wooden block has been pushed in front of the observer which then detects this and turns on the redstone lamp for a few seconds.
It has got many usages. For example, you can build stone generators and hidden door openings.