When you run a site managed by more than a user (for example when you have two or more administrators or you have a handful of editors), having an overview of what is happening in the blog is important.
Monitoring is not meant to keep an eye on your collaborators, but to be able to diagnose what caused problems when they arise (and problems always happen).
I found myself in this situation recently and I looked at the WordPress.org repository for a plugin able to log the administration side actions taken by allowed users.
There are many of them: WP Activity Log, WP User Activity, User Activity Log, Activity Log (and the list grows).
To be honest, I picked one without comparing too much the features with alternatives and I installed WP Activity Log. I find it very complete to my needs.
I’m not an affiliate.
With a simple configuration wizard, it setups in a few seconds. You can choose between two log levels, one very very fine and one more coarse. Later, in the settings, you can change that and even select one by one the events you want to log. There are tens of them!
The log is very easy to read and with summary and detailed information.
A lot of features are available with the premium version but for basic usage, the free version is far enough.
Of course be aware that a monitoring plugin does not save you from damages, like article deletion and configuration changes: if you need fine control over what people can do, other than the known “user role” you should install a plugin to define, change e manage users’ roles and capabilities.
Final consideration. Coming from years of Java development in enterprise contexts what I miss more about WordPress is the logging capability. This plugin (or a similar one) is a must-use on blogs managed by more than one person and probably is can be complemented with other logging plugins to know what is happening in your site.