Some mailbox providers analyse how recipients interact with emails and base filtering decisions on how the majority of users react. This typically means that if you are sending emails that the majority do not open, mark as spam or delete without reading then your all of your emails are likely to be moved to the spam folder or worse.
Positive engagement indicators, such as opening the email, adding the sender to the address book or taking an email out of the spam folder contribute to emails being delivered as normal to the inbox.
The bottom line is that engagement based filters demand that senders deliver content that recipients actually want to read.
Get in touch if you need help understanding engagement filtering.