Player enters door trigger zone. If he has the red key, the door opens.

1. Objects
There's four objects in this example.

2. Animation
The "door" object has an "open" animation. By default it plays automatically.

3. Switch
Every node has an option Switch that can be turned on in its Inspector, represented by the black dot.
A Switch is either on or off. Only if the Switch is on will the node be called, in this case the animation played.
Potentially, a switch can be configured to be continuous (keep calling as long as the switch is on) or as a trigger (call once when flipped on).

4. Switch expression
A switch can be any expression that returns a boolean.

In this case, the switch is turned on if the player object is in the zone object and if the player object has the key object.
The "in" and "has" nodes are Operator nodes that could simply be options the user chooses from a list, or expressions entered in code (or visual logic).

5. Connections
When the children of the Operator nodes are dragged out, you can see their connections with other objects.