The Bitcoin Difficulty Ribbon is based on Willy Woo's implementation and it gauges the selling pressure experienced by Bitcoin miners.
The Bitcoin Network Difficulty is a measure of how complicated it is to find the next block, compared to the ease of finding the easiest possible one.
When the difficulty decreases it's a hint that the mining sector is experiencing difficulties and the longer it contracts, the more miners might get out of business.
During these contracting times, the Difficulty Ribbon narrows and becomes a significant proxy to recognize cycle bottoms.