This means: other developers working with you has made changes on the same page or microflow as you made your changes locally.
Most of the times when you update your code, the changes will be merged. But sometimes, SVN (team server of Mendix) cannot merge the code and will report merge conflicts and this has to be fixed manually.
Steps to follow:
You can see the conflicted pages or microflows in the changes pane of the mendix studio pro.
You can open the changes tab by clicking view button at the top and select changes to open changes tab at the bottom
I hope this helps you