A high ratio means more puts are trading, traditionally read as fear and therefore as a contrarian buy signal. The measure can be built from volume or open-interest, from all contracts or from small-lot retail orders only, and the versions disagree.
The weakness is structural: the data cannot tell buying from selling. A surge in put volume may be investors buying protection or premium sellers writing puts into a decline, and those are opposite sentiments producing an identical reading.
Example: XYZ shows a put-call ratio of 2.4 on a down day. That could be holders hedging, or it could be income traders selling $45 puts into weakness. The ratio is the same either way, which is why the index-wide version is the only one most traders bother with.
Related: option-volume, open-interest, options-flow, max-pain