This is a log of measurements along a well bore (a well-log). You might have encountered this acronym
PO4C in your log evaluation software or any (ASCII) log files provided by the logging company. The code stands for
'Partial porosity from MRIL (single element)'. The usage of this log is well established. The values of this log represent the contribution of porosity from a given "bin" (with certain characteristics) to the total porosity; e.g., in nuclear magnetic resonance data processing, the "bins" typically correspond to different pore size ranges. At a higher level, this can be seen as a measure of the apparent porosity. This can be explained as being the initial estimate of porosity, normally from logs, not corrected for the (often large) effects of clays, gas, etc. Such estimates should only be used for qualitative interpretation of clean, gas-free intervals.
So basically this is a measure of the pore volume per unit gross volume. Porosity is determined from measurements on cores or interpreted from logs. At a more general level these values are measures of the volume fraction. Better said, the ratio of the volumes occupied by two components, or by one component to the total volume of the system.
Data for this log is produced by a tool that uses a nmr logging technique and can be used to investigate the porosity of the formation. Similar logs are listed below: