Chain Actions is a setting that is useful in campaigns which have multiple tools enabled. (It serves no purpose for single-tool campaigns, so you can safely ignore it!)
For example, in this 5K campaign, we want to make sure everyone who registers for our run will be prompted to take another available action immediately after registering: Start Fundraising.
Here is what the Thank You page looks like for the Registration tool that has "Start Fundraising" as a chain action:
You can configure Chain Actions by going to Tool Settings > pick the tool after which you'd like to encourage another action > click the first Settings menu > scroll to Thank You Page Options > check the box for the additional action(s) you'd like to encourage.
Here is what the settings area looks like for our example campaign: