A rule starts with an event.
Everything a challenge does starts when something happens in the game: a player breaks a block, a mob dies, a chunk loads, the clock ticks. On the canvas the event is the first step of a rule, and the steps wired after it run in order.
Filters narrow it.
Most events take filters: which blocks, which mobs, which items, which dimension. Left empty, a filter matches everything. A filter takes several values, and a tag such as any logs stands for every block in it.
What an event carries.
Each event brings fields the steps can read: the player, the place, the block or the mob it was about, a number such as the damage. The inspector offers them wherever a value of their type fits, and text shows them as placeholders such as {block_type}.
Timers and ticks.
A timer runs its steps every so many seconds, or once after a delay. A tick runs them every few game ticks, for the few rules that must act every moment, such as pulling things toward a player. Neither has a player of its own, so their steps say who they are about.