The /default tag is how Limio defines the homepage of your website. In effect, "/default" and "/" is the exact same thing. Let's explain.
Say you have a shop made available on your domain, such as www.limio-shop.com. In the Limio app, you then create a new Campaign and set the tag for that Campaign as /default and publish. The full URL for this Campaign will now be www.limio-shop.com/default AND www.limio-shop.com. It's now your homepage.
Now, /default can also be used for Limio Journey. If you set /default as the starting point of a specific journey, everyone coming to your site will hit that journey and its conditions. So take extra care to review the journey, make sure all the Campaigns in the Conditions column are live (you can just click on the 'Show campaign details' to get information). If all looks good, then change the starting point to /default and save. This will immediately set the /default to that journey, which you can test by going to your domain such as www.limio-shop.com.
Finally, /default is also the page Limio will default to if Limio can't find a certain URL - in effect, it is also the 404 page.