RGB measurement

[troywander]

        lable        area         mean stddev min max
1 3.jpg   580800        6.989        18.347 0 255
2 Red      580800   17.408 19.879 0 255
3 Green     580800   1.608       18.275 0 255
4 Blue     580800         1.798 18.362 0 255
i want to measure the intensity of red fluorescence of that  image by imageJ.
i used measure under analysis  and get a mean value 6.898 in the 1st line. what does this 6.898 mean.
and i downloaded RGB measure plugin to do it again and get 17.408. what does this mean.
and what the unit of this value?