TLDR: Interactions are when a user explicitly interacts with your bot through a slash command, context menu or component. They must be responded to through any of the reply to interaction blocks.
What are they?
There are two different ‘types’ of events that you will encounter when using inventor, those being interactions and ‘gateway events’. If we use the slash command trigger and the message sent trigger we can view the difference as this.
Interactions
A slash command (interaction) is like the user is starting a conversation with your bot, you then respond to them (like you would in a conversation) through a reply to interaction block. If you don’t then they are left without a reply and get an error, they started a conversation with your bot and got ignored.
Gateway Events
A message sent trigger (‘gateway event’) however is like your bot overhearing someone saying something and then talking to them through a send or edit message block. The user did not explicitly interact with your bot, instead your bot sent a message by itself after seeing a user send a message. Your bot may not even send a message and just silently complete an action. It is worth noting that because the user did not explicitly interact with your bot you cannot use ephemeral messages.
Summary
To summarize an interaction is when a user explicitly interacts with your bot through a command, context menu or component (button, drop-down, etc) while other ‘gateway events’ don’t require the user to interact with your bot in order to do stuff. You must respond to interactions while you don’t have to for ‘gateway events’.