it is the lazy inaccurate way of calculating how many blocks are going to be mined in N days.
there is no guarantee to have 1 block every 10 minute, but usually the average is that. 1 day is 24 hours and 1 hour is 6x 10 minutes so we can see 6*24=144 blocks in one day.
a better way is to extrapolate the data that we have from previous weeks, at least for 5 difficulty adjustment period. i tried doing that using blockchair API but faced some problems and gave up