SOPR, the Spent Output Profit Ratio, divides what coins were worth when they moved by what they were worth when they last moved before that. It looks only at the coins that moved in the period.
Above 1, the coins moving today are on average in profit. Below 1, they're being spent at a loss.
The 1.0 line
People don't like selling at a loss. In a healthy uptrend, dips towards 1 tend to dry up because holders refuse to sell below cost, so 1 acts like a floor. In a downtrend the same psychology flips it into a ceiling: rallies back to 1 give under-water holders their chance to get out even.
Adjusted and cohort versions
Adjusted SOPR removes very young coins, which often are just a transfer between someone's own wallets and pull the figure towards 1. Splitting SOPR by holder age is more useful still: long-term holders spending at huge profit tells a different story from new buyers selling at a small loss.
Relative unrealised profit
NUPL, net unrealised profit or loss, is SOPR's static cousin. Instead of what was realised today, it asks what the whole market would realise if everyone sold now.