You have "on Tick" - blocks placed there are executed on every tick. Well, the ones at the top (where no blocks are connected above them). These blocks should be conditions, filters... anything that asks for certain condition before taking some actions.
Tick comes -> Check some conditions -> Do some actions.
For example, conditions might be:
No trade is running -> Check if MA5 is above MA10
and the action can be:
Buy now
Also there is a category for working with indicators. This category is named "Indicators" and it contains some blocks that are some kind of complex Condition block. Also Condition can be used to check what it has to be checked.
Indicators are different in how they communicate with the EA. They communicate with their buffers. On each of these buffers you have current value and values for each bar in the history where the indicator is drawn. A buffer is an array... if we have to say it in programming language. And "Indicators tester" is used to just see what is in a single buffer, at least to know if it's something meaningful, not crap (because there are many indicators with crap on their buffers).