TL;DR: free from copyright
A work is free from copyright when nobody owns it any more: its term has expired, it was never protected, or its owner waived the rights with CC0. You can then copy, change and sell it without asking. In the US that covers everything published before 1931; in most of the world it covers works by authors who died in 1955 or earlier.
Most "free" downloads are something else. Unsplash, Pexels, Pixabay, Fab and BlenderKit's royalty-free assets are free to use but still protected, under the site's own licence. For 3D work, the safest sources are CC0 libraries such as Poly Haven, ambientCG, Kenney, Quaternius and Smithsonian Open Access. This guide shows how works become free, what enters the public domain on 1 January 2027, where to find assets that really are free from copyright, and how to use them in Blender without surprises.
Mix free assets with your own work, and prove which part is yours. BlendedBoris is a free plugin for Blender that records your 3D model on the blockchain and gives you a lifetime ownership certificate. It creates a digital fingerprint that links the model to you, so when your scene combines CC0 textures with your own modelling, you have dated proof of authorship for every version you made.
👉 Get the free BlendedBoris plugin