The baboon is adept at climbing trees, rocks and various steep formations. This is due to the fact that it learns to climb from infancy and does so throughout the day time when it can see where it is going and what to hold on to. But even if it became dark the baboon would still be able to climb the steep slope due to the experience it has honed by doing the same in the day. From this example our elders saw the lesson that when one gets used to doing things, one can even do them under difficult circumstances with ease. In other words, we do easily that to which we are accustomed.
The proverb is used to explain any situation where one accomplishes something with ease due to experience when others doubted them. It can also be used to advise on the kind of person who should do a routine task that has been made more complicated, by stating that the experienced is the capable one.