What does spreading factor mean?

Hey guys,

I already checked the forum and the Internet for an explaination of the Spreading Factor (SF). I found out that LoRa automatically choses a factor between 7 and 12. Higher SF means lower data rate, higher sensitivity and longer range. Lower SF means higher data rate, lower sensitivity and shorter range.

Unfortunately I could not find an answer to the question: what exactly is the spreading factor? How does it physically change the signal? I read that SF means the number of transmitted bits in one symbol, but that does not make sense to me - why would the bitrate decline if I have more bits in one symbol?

Another explaination could be that SF describes the number of chirps in one symbol, meaning that with SF12 you have more chirps in the symbol, thus needing more time to transmit the symbol → lower bitrate.

Any answer / link / quote would be a great help! Thank you!