A sequence is a list of actions which can be triggered via switches, sensors and other means.
An action has two main parameters, a delay in seconds and a command to send. Each action will count its delay before executing the command.
When the sequence is run because it was triggered by a profile change it will start the sequence at the action that is selected as the profile change action.
When a sequence is triggered again whilst it is already running it will run the override behaviour for the current action that the sequence is on which is usually going to a different action but can also be ignored so the sequence will continue to run from where it currently is.
The delay is a given number of seconds that the sequence will wait before performing the given action.
The conditions specify what requirements must be met in order for the action to run otherwise it will continue on to the next action.
The action is what will happen when the sequence reaches this point after the delay time has been exceeded and the conditions for this action are met. There are many types of actions that can be set in a sequence which allows for many scenarios that sequences can be useful for.
Once the action has been run(or skipped by not meeting conditions) it will run the behaviour specified in the post action.