When you post a comment, video, photo or any other type of user-generated content (UGC) on a website or app owned by a service, this service needs your permission to legally use and share that content throughout their platform (or beyond). For that, you need to grant them a license. Depending on the characteristics of this license, the service may have some or all of these rights over your UGC:
This means the license will remain in effect even after you leave the service by deleting your account. Your content may still be publicly displayed by a service you no longer use, that’s why users need to be cautious before granting this kind of license.
Transferable and sublicensable license
This means your content may be shared with a third-party if:
the service you granted the license to is acquired by somebody else (transferable license)
the service wants its affiliates to use the content (sublicensable license)
A royalty is a payment to the owner of a content to have the right to access, use or share it. It applies to content subject to intellectual property rights (e.g. copyright). When you grant a royalty-free license to a service, you allow them to use your UGC without paying royalties.
In laymans terms, when you license your content under a non-exclusive term, it means you are free to share the same content in whatever other platform you desire.
Non-exclusive use is the ability for the licensor (i.e you, the user in this case) to share their content through other means.
If a service binds you to an exclusive license, you'd be technically forbidden to upload/share your content in any platform other than that one.