Ladders -- as in, ladders of engagement -- are an automated series of actions an activist goes through if they meet certain conditions, called triggers.
Activists enter ladders by triggering conditions on rungs. An example of a trigger could be subscribing to the email list or opening an email. Activists then move down a ladder's steps, which can include actions (which modify or interact with an activist, such as sending them an email), decisions (which evaluate an activist based on conditions such as having a certain tag and move them down different paths depending), and waits (which pause an activist for a certain amount of time before moving on to the next step).
With ladders you can design simple or complex automated paths for your activists to take. For example, you can automatically send activists a series of emails welcoming them to the list over the course of a week after they subscribe. (See here for more details on this example.) Or you can design a full campaign plan for your activists to move through, from signing a petition to writing a letter to donating to the campaign, all automatically, with different messages depending on what actions the activist personally takes. (See here for more details on this example.)
Overall, ladders allow you to personalize email automatically to activists depending on their engagement with your organization and parameters you set up.