MultiBit is under active development and gets regular updates containing bug fixes and new features.
Whenever the application starts it checks against MultiBit.org for any new updates and if one is found an update process is triggered. This will give you the option to upgrade at your convenience.
The upgrade process will usually simply overwrite the old version with the new one. Once the process has completed you will be able to continue working just as you were before.
The updater process works as follows:
When the new version of MultiBit is launched it may perform a wallet update to introduce new features or configuration information. During this process any existing wallets will be automatically backed up.