Protection from a quality grants four specific shields, often remembered by the acronym DEBT: the protected creature can't be Damaged, Enchanted/Equipped/Fortified, Blocked, or Targeted by sources of that quality. The 'B' in DEBT means a creature with protection from white cannot be blocked by white creatures.
This is stated explicitly in CR 702.16b: a protected creature 'can't be blocked by' creatures with the specified quality. If a white creature is declared as a blocker for a creature with protection from white, that block is illegal and must be undone during the declare blockers step (CR 509.2).
Note that the protection is on the attacking creature. It is the attacker that has protection from white, and as a result, white creatures on the defending side cannot legally block it — not the other way around.
Example: Your White Knight (a white creature) attacks. Your opponent controls a Serra Angel (also white). If White Knight had protection from white, Serra Angel would not be a legal blocker for it. Your opponent would need a non-white creature to block White Knight, or take the damage unblocked.
The blocking restriction is absolute: no ability can override protection's blocking prevention unless a rule or effect specifically says it can block as though the blocker didn't have that restriction (CR 702.16b, 509.1b).
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.