Answer: It is everything that people introduce in the religion that did not exist at the time of the Prophet (may Allah’s peace and blessings be upon him) and his Companions.

We should not accept it; rather, we should reject it.

Because the Prophet (may Allah’s peace and blessings be upon him) said: "Every religious innovation is an error." [Narrated by Abu Dāwūd]

Examples of religious innovations are: adding to worship, like adding a fourth-time washing to ablution, and celebrating the birthday of the Prophet, which was not reported from the Prophet (may Allah's peace and blessings be upon him) and his Companions.