When a comparator is placed against a container, it gives off a signal when there are items inside that. This is often used as a simple on/off switch for a circuit emptying said container, but it can do more.
The strength of the output depends on how full the container is, starting at 0 when it's empty to 15 when full to the brim, with a stepped scale in between. In the demonstration below, you can see the output strength in the bottom right corner.
The maximum stack size of items inside the container is also taken into account, so one non-stackable item increases the output by the same amount as 64 of most blocks or 16 buckets or signs.
The comparator also interacts with some non-container blocks with outputs that don't always go all the way to 15. It can for example be used to check how much honey is in a beehive, how much of a cake has been eaten, how full cauldrons and composters are, what the rotation of an item in an item frame is, which music disc is playing in a jukebox, or what page a book on a lectern is currently opened on. (Again check the signal strength in the bottom right in the demonstration.)
If you don't want an output until the input reaches a certain level, you can put the comparator on subtraction mode and lead a second signal into its side. The output will be then be the main input minus the side input, with a minimum of 0.
In the example below the chest is full, leading to an output of 15. But because a lectern outputting a strength of 6 is subtracted from that, the final output is only 9. And if the chest were to only contain enough items for an output of 1-6, the final output would still be 0, allowing you to gather a few stacks before triggering any signal at all.
Even in normal mode you can run a second input into the side. Then the input in the back will only be forwarded in full when it's stronger than the one into its side.
