You are not the only person working on the project, right? This means that your colleagues are also making modifications to the project's data. To stay up to date and to fetch the modifications committed by others, you should run svn update command in your working copy. As a result, your working copy will sync with the repository and download the changes made by your colleagues.
Shorthand for svn update is svn up.
It is a rule to run svn update before committing your changes.