BOINC's unit of credit, the Cobblestone (named after Jeff Cobb of SETI@home), is 1/200 day of CPU time on a reference computer that does 1,000 MFLOPS based on the Whetstone benchmark
Eventually, credit may reflect network transfer and disk storage as well as computation.
Credit has no monetary or other value; it's just a measure of how much work your computers have done.
Total credit: The total number of Cobblestones performed and granted.
Recent average credit: The average number of Cobblestones per day granted recently. This average decreases by a factor of two every week.