Marketing automation is the practice of letting software handle the predictable parts of your marketing - so the right person gets the right message at the right time, without your team manually pushing send.
Common building blocks
Email sequences triggered by behavior, lead routing rules, dynamic content based on segment, scheduled social and ad campaigns, and reporting that connects activity to revenue.
Signs you're ready
You have a steady flow of leads, a working sales process, and a CRM that's at least partially set up. You don't need a huge team - in fact, automation usually pays off most for small teams trying to act bigger.
Common mistakes
Trying to automate before you understand the underlying process, sending generic blasts instead of relevant messages, and treating automation as set-it-and-forget-it instead of an asset that needs tuning.