Alchemy.Events.on_reaction_add

You're seeing just the macro on_reaction_add, go back to Alchemy.Events module for more information.
Link to this macro

on_reaction_add(func)

(macro)

Registers a handle triggering whenever a user reacts to a message.

args : snowflake, snowflake, snowflake %{"animated" => boolean, "id" => integer, "name" => String.t}

Receives the id of the user that reacted, the channel_id where it happened, the message_id and the emoji