NFL Rule Book Section 4 Article 8

Throughout the period on game-day that a player is visible to the stadium and television audience (including in pregame warm-ups, in the bench area, and during postgame interviews in the locker room or on the field), players are prohibited from wearing, displaying, or otherwise conveying personal messages either in writing or illustration, unless such message has been approved in advance by the League office.
Items to celebrate anniversaries or memorable events, or to honor or commemorate individuals, such as helmet decals, and arm bands and jersey patches on players’ uniforms, are prohibited unless approved in advance by the League office. All such items must relate to team or League events or personages. The League will not grant permission for any club or player to wear, display, or otherwise convey messages, through helmet decals, arm bands, jersey patches, or other items affixed to game uniforms or equipment, which relate to political activities or causes, other non-football events, causes or campaigns, or charitable causes or campaigns. Further, such armbands and jersey patches must be modest in size, tasteful, noncommercial, and non-controversial; must not be worn for more than one football season; and if approved
for use by a specific team, must not be worn by players on other teams in the League.