When you decide to secure your marketo landing pages using SSL, the final step will be Marketo consulting "cutting over" the server so that all of the landing pages redirect to their SSL versions.
When that happens you need to worry about mixed content.
What is that? Good question. Mixed content is non secure content (HTTP) served within a secure page (HTTPS), and it comes in two types, "active" and "passive". Passive mixed content such as the following will generall still render (depending on browser and version, but will result (again depending on the browser) in a warning]
Active mixed content will generally cause a larger problem with the page, such as the visual aspects not rendering (CSS)
All mixed content should be addressed, but only the active content will actually break the pages.
The official definition of mixed content as defined by the W3C is here --> Mixed Content