As you use the application, you create data such as recipees, foodmarks, DRI values, your daily menu, etc. All these are stored in your device, using a standard browser feature called "local storage".
You may have adjusted your browser settings to automatically delete local storage for a website when you exit the browser. I have done this for privacy reasons, because websites may store unique identifiers in local storage, and use them to track me (like cookies). If you do this, you can also specify an exception for the websites that you allow to store data in local storage.
Your browser may also automatically delete data from local storage to save space or for other reasons. To prevent this from happening you have to let the browser allow this website to use persistent storage.
You can check the status of this setting and enable it via the Data page.
If persistent storage is not enabled, the PERSIST button will be enabled. When you can press it, the browser will ask you if you want to do this. The website cannot enable it without your explicit permission.
You can remove this permission from your browser settings.
This application is not very useful without persistent local storage. It will work fine until you exit the browser (or the application windows). Then you will lose all the data you entered.
The application also has ways to export and import your data to/from local files. It's a good idea to do this periodically. For example, you may have spent a lot of time researching and entering DRI values. To safeguard against losing this work, save them to a local file.