As the site owner, you would like to send reminder emails to the members if they have not logged in for a long time. This is important since it helps catch some users who otherwise would leave the site permanently.
Log into your AdminCP and click on Members > Inactive Members
In this page, you can get a list of Inactive Members and send the reminders message to them.
This feature will send emails with the email content of the phrase "user.mail_inactive_users". You may like to go to Globalization > Phrases to search and update this phrase if you want.