Zapier is a sort of connector or Middleware between different apps. Examples of supported Apps include:
(A full marketplace of Apps can be found here: https://zapier.com/apps/integrations)
Zapier essentially allows a person with no technical knowledge to set up a data connection between two Apps so that when a particular event happens on one App, data is pushed into the other.
When a new Email address is captured via a Hubspot Blog, push that email address into a particular MailChimp mailing list.