Blocks placed under "on Tick" are executed on every tick - again and again and again, maybe few times per second. At least first ones above others. A block, when executed, does what it has to do and it can pass or not depending on what it does (see it's description for information about that).
Some blocks are conditions, others are filters (like "No trade is running")... and filters are conditions on steroids.
Other blocks are actions, like "Buy now", "Sell now", "Close trades" and all the others that do changes somewhere. "Draw arrow" can be also called action block, but it is more for debugging (seeing what is going on).
Connect blocks like this: Conditions, Filters => Actions
If you don't know what some block does, there is an information about it. Hold your mouse over the block in the list, or if you have it as a block - right click and hit "Information".