The idea in one line

Most defences hide or blur your details. Tor Browser does something different: it tries to make every Tor Browser user report the same details, so there is nothing unusual to latch on to. The EFF describes it the same way: when a browser's characteristics match all other copies of it, it cannot be singled out (EFF).

What Tor Browser does

The Tor Project lists these measures (Tor Project):

Fewer buckets. The goal is to reduce how many distinguishable groups exist for each measurement. The project says it is practically impossible to make all users identical, so it narrows the spread instead.
Letterboxing. The content window starts at a size rounded to a multiple of 200 × 100 pixels. When you resize or maximise, grey margins keep the visible area close to one of those standard sizes.
No choice of platform. You cannot pick which operating system Tor Browser pretends to be. The project says this is intentional: a choice would split users into smaller, easier-to-spot groups.
NoScript integration is listed among its anti-fingerprinting features.

Why the margins appear

The grey bands around pages are letterboxing at work. Window size is one of the details a site can read (our browser check shows yours). Rounding it means thousands of people share the same value instead of an exact size that is almost yours alone.

Habits that work against it

These follow from the design rather than from a rule the Tor Project page states:

Adding extensions or changing settings makes your copy differ from everyone else's. The EFF gives the general warning that changing one setting at a time can make a browser more identifiable (EFF).
Logging in to an account tells the site who you are, whatever your fingerprint looks like.
Making the window exactly fit your screen by other means defeats letterboxing.

Compared with other approaches

Approach	Example	Trade-off
Look identical	Tor Browser	Works only if you leave the defaults alone
Add noise	Brave, Firefox canvas protection	Noise can be noticed (EFF)
Block known scripts	Firefox Known Fingerprinters	Unknown scripts are not caught (Mozilla)

More on the other two: Brave farbling, Firefox protection, and the overview of fingerprinting techniques.

Limits

The Tor Project page we read does not claim perfect protection and does not list warnings; it describes design goals. The Tor Project also notes that Tor Browser often makes your connection appear to come from an entirely different part of the world, which can lead some sites to lock or question your account (Tor Project).

Outside this page: Installing Tor Browser, onion services and security levels. The Tor Project's own support pages cover them.