This is essentially a quick rundown of open-source software licensing. The notice is saying that the TikTok app uses some software that is owned by Facebook, but that Facebook has irrevocably licensed the software such that anyone (including you or TikTok) can use and distribute that software for free, provided they follow the few rules in the BSD 3-clause license, which has three clauses: 1) include these three clauses with any source code copy of Facebook’s software, 2) include these three clauses in the docs bundled with any compiled app that uses Facebook’s software, and 3) do not use Facebook’s name in a way that implies an endorsement or affiliation with Facebook.

TikTok can continue using that particular version of Facebook’s software until the heat death of the universe, and Facebook can never come back later and demand payment from TikTok or you or anyone for that software. But Facebook is still considered the owner, because they retain the right to relicense the software under different terms, perhaps with a license that doesn’t require including the copyright notice, for example. Likewise, Facebook has the right to sue to enforce the BSD 3-clause terms against anyone who isn’t abiding by those terms. But it looks like TikTok is abiding, since they posted the full BSD 3-clause terms, so Facebook can’t complain.

Note: Facebook could change the license for later versions of the software, but any versions prior would be unaffected. Integrating any software commercially always requires checking the license terms, and while open-source software has fairly standardized terms, diligence is still important and licenses do occasionally change.