TSL2591 sensor value calculation
This sensor has two integrating light sensors with different wavelength responses.  The purpose for this setup is to allow the sensor to simulate the spectral response of the human eye.  The first chart below is the human eye's response, the second is the response from the sensor datasheet.  The calculation at the end with the coefficients is to convert the sensor outputs to lux.  Lux is lumens per m^2, and a lumen is defined according to the response of the human eye.





cpl - 
The general idea is that you set the gain for the input, then set the amount of time the sensor collects light. The longer time you select, the more light you will get.  Once this is complete this, you will have a certain number of ADC counts.  This has to be scaled by that gain and integration time. cpl appears to be that "counts per lux" scaling.

TSL2691_LUX_DF - This is hard to say for certain given the information, but it appears to simply be the default lux per count given a gain of 1 and an integration time of 1 ms.

If you want more info about the sensor, go to the AMS website and find white papers there.  They are under the sensors themselves.

For radiometric physics, the Georgia State University Hyperphysics resources are fantastic:  http://hyperphysics.phy-astr.gsu.edu/hbase/ligcon.html#c1