The metaphor here is that if you start by putting sand into the jar, you will not have room for rocks or pebbles. If you spend all your time on the small and insignificant things, you will run out of room for the things that are actually important.
Really bad metaphor. Here the author think that rocks are equal to important things in life. Small sands are not important in life. However, every small steps counts, if you imporve 1 % per day, this will have great impact in life.
