Well, it can mean to sharpen a saw. Use the dictionary to look up "sharpen" and "saw". It is also an idiom that means to "work smarter, not harder". If you try to cut down trees with a dull saw, you may spend 1 day cutting down one tree, so 10 trees => 10 days. Now, let's say you spend one day to "sharpen your saw". OMG, I've lost one day. Yes, but NOW you can cut 3 trees per day with a sharper saw, so your overall time with a sharpened saw is 5 days maximum. So this idiom is saying that sometimes you have to stop and do some prep or refit work (wasting time now) so that you can proceed faster later (saving more time later than you lost now).

It's also a concept in Stephen Covey's 7 Habits of Highly Effective People. Basically it means to take time to take care of yourself. https://www.stephencovey.com/7habits/7habits-habit7.php