A behaviour is sometimes referred to as a ‘blocking’ behaviour. Technically, the execution of a behaviour should be non-blocking (i.e. the tick part), however when it’s progress from ‘RUNNING’ to ‘FAILURE/SUCCESS’ takes more than one tick, we say that the behaviour itself is blocking. In short, blocking == RUNNING.
flying spaghetti monster

Whilst a serious religous entity in his own right (see pastafarianism), it’s also very easy to imagine your code become a spiritual flying spaghetti monster if left unchecked:

  _  _(o)_(o)_  _
._\`:_ F S M _:' \_,
    / (`---'\ `-.
 ,-`  _)    (_,

A key feature of behaviours and their trees is in the way they tick. A tick is merely an execution slice, similar to calling a function once, or executing a loop in a control program once.

When a behaviour ticks, it is executing a small, non-blocking chunk of code that checks a variable or triggers/monitors/returns the result of an external action.

When a behaviour tree ticks, it traverses the behaviours (starting at the root of the tree), ticking each behaviour, catching its result and then using that result to make decisions on the direction the tree traversal will take. This is the decision part of the tree. Once the traversal ends back at the root, the tick is over.

Once a tick is done..you can stop for breath! In this space you can pause to avoid eating the cpu, send some statistics out to a monitoring program, manipulate the underlying blackboard (data), … At no point does the traversal of the tree get mired in execution - it’s just in and out and then stop for a coffee. This is absolutely awesome - without this it would be a concurrent mess of locks and threads.

Always keep in mind that your behaviours’ executions must be light. There is no parallelising here and your tick time needs to remain small. The tree should be solely about decision making, not doing any actual blocking work. Any blocking work should be happening somewhere else with a behaviour simply in charge of starting/monitoring and catching the result of that work.

Add an image of a ticking tree here.