What does it mean to follow someone?

This means to subscribe to all of the posts from a particular user in Expressom. In your dashboard in the Activity Feed, you will see all of the posts from the users that you follow.

To begin to follow a user is easy. Just go to their profile and click on the icon that has a + sign with a profile next to it at the top right. It will say Follow username. If you later decide that you no longer want to get user updates from this user, go to his profile and click on the same icon that says Stop following username.

Moreover users can monitor groups and tags.

How can I see posts of all people who do not follow?

One of the filters in the left part of dashboard states Everybody. When you click on this filter you will see all public posts of all users in Expressom, whether you follow them or not. It’s a useful way of finding new users and groups that might be interesting to you.