Arf, look at my yesterday's thread "comments and rumblings from a new player", the highest authorities here discussed just that point
In a nutshell, each set of 3 lines says :
Line 1 = nr of pop for following effect to occur
line 2 = % modifier of resources prod for that nr of pop
line 3 = % modifier for Startyards construction rate for that nr of pop
At first the count of modifier is given to the program.
So in unmodded game there are 12 "levels", with 0-99 M people giving 100%/100% modifier, and 10 B+ pop giving 200%/200%
These numbers are highly unrealistic-looking, and are modified in some mods, say Proportions