SOPR is an Abbreviation of The Spent Output Profit Ratio (SOPR). it is computed by dividing the realized value (in USD) divided by the value at creation (USD) of a spent output. Or simply: price sold / price paid.
In fact, this index shows whether investors are in profit or in loss? See the link below for more information on SOPR:

In this analysis, we want to examine the SOPR index for Bitcoin .
As can be seen in the chart, the SOPR is on a downtrend. This shows that investors are out of profit mode and now this index shows the number 1, which indicates the lack of average profit and loss of investors.
The SOPR does not currently indicate an end to the downtrend and is likely downtrend continue in the future