Many Contentful accounts use spaces for staging. Assuming you have a source space and a destination space, you can keep published content synchronized over time. Any draft or unsaved content will not be synchronized.
Each time you run the tool it stores a synchronization token so only new Entries and Assets get copied, and so that deleted items can also be deleted on the destination space. See the Synchronization documentation for more details.
Content Types will always be updated as they are not retrieved by the synchronization API, and Content Types which don't exist anymore in the source space will be deleted in the destination space as well.
If you make any manual changes in the destination space, be aware that this tool will overwrite any changes you've made to entities with the same ids as those existent on the source space.
Also, avoid creating new Content Types and Entries in the destination space. This tool is intended to be used with a workflow where you create content on one space and then regularly copy it somewhere else in an automated way.