FCC part 15 "must accept interference from other sources": what does this mean?

FCC Title 47, Part 15 section 5.b should clarify this for you:

(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions ... that interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiator, by industrial, scientific and medical (ISM) equipment, or by an incidental radiator.

In other words, to be FCC listed under part 15, your device must operate normally when subject to normal, authorized RF emissions from other sources.

You can drill down to the specific requirements under part 15 by starting here:

http://www.fcc.gov/encyclopedia/rules-regulations-title-47

"Interference" in this context applies to one, radiated RF signal affecting the reception of another radiated RF signal. The interference is not produced within the interfering transmit system, but as a result of the radiated field intensity it produces at the receive location(s) of other transmitted signals.

Transmit systems whose operators and equipment are specifically licensed/certified by the FCC, and are operating as permitted by their station license are protected from such interference from "Part 15" unlicensed operators and systems — even when those unlicensed operators/systems strictly meet the technical requirements set out for them in FCC Part 15 rules.

This FCC protection from interference applies only to certain electronic devices using the RF spectrum. For example, the user of an audio amplifier in a home stereo system may hear the programming of a licensed AM broadcast or HAM radio station located physically nearby. But there is no FCC recourse. S/he must either accept that interference, use an amplifier that doesn't exhibit that issue, or relocate that stereo system far enough away from the transmit system so as not to be affected by its legal operation.