Marketing automation is where you implement systems to do your marketing for you, based on the behaviour of your audience.
The best way to explain marketing automation is to give an example.
Suppose your business is the sale and installation of spa pools.
Mary visits your website. She has a look around, and visits the product page for one of your spa pool models. This page also happens to have a form which offers Mary a free guide to the top five things to be wary of when buying a spa pool. All she has to do is fill in her first name and email, and she'll get an email with a link to the free guide - which she does.
Your marketing automation system immediately adds her to your database, marks her as a "prospect", and assigns a lead "score" of 10 points. It also sends her a thank you email that also includes a link to the downloadable PDF download of the free guide.
Mary receives the email and immediately clicks the link to download the PDF. Your marketing automation system records the fact that she downloaded it, and assigns another 10 points to her score.
Three days later, your marketing automation system sends Mary another email. This one checks that she received the PDF link, and hopes if she found it