Tor: How does Tor Browser reduce fingerprinting?

Short answer

It tries to make many users look alike by limiting or standardizing characteristics such as screen size, reported browser details, fonts, and site data. Letterboxing reduces the precision of window-size measurements, while site isolation limits cross-site tracking. These defenses reduce distinguishability; they do not make every user identical or protect information you choose to reveal.

Sources

Tor Project: Fingerprinting protections.