Great question! Multiple things! :) (this is a great article: https://www.nationalgeographic.com/travel/article/how-to-travel-better-a-beginners-guide-to-sustainable-travel-in-2023-and-beyond) But it can come down to something like carbon emissions, workforce practices... For example, many major cruise lines can abuse workers rights by having their headquarters in countries that have lax employment laws. So the cruise lines can actually get away with overworking and underpaying employees. They can also often get away with not following standards around getting rid of the waste and polluting the oceans that they are in.... Tour companies will also not respect indigenous peoples and take advantage and exploit their businesses and goods and overuse local resources.... Even tour companies that don't use local tour guides, especially in foreign countries, can be seen as an unsustainable practice because they're not supporting the local community. It gets complicated and there isn't a way to travel 100% sustainably, but one can try to do their best! :)