When starting a new game, we often have a tendency to try to fill out as much detail as possible. We may create a detailed family tree of the royal family, work out exactly how magic works, or how the Gods came to be. These are the details that interest and excite us, and hopefully they do the same for our players!
Though it sometimes turns out that the players may never learn the details of the kings family tree, if you had fun creating that part of the setting, then it is no loss having spent time on it. After all, fleshing it out made you more invested in the setting! Even so, sometimes it is better not to fill in every detail. By leaving things unwritten, you avoid painting yourself into a corner, and you have more time to devote to other, more important aspects of the setting.
For example, if you are starting a low level D&D campaign, there is no reason to detail exactly how magic works, unless it directly impacts the way PCs cast their spells. Even if it is interesting theoretical knowledge for the party wizard, low level characters are unlikely to interact with the fundamental nature of magic beyond an academic acknowledgment.
And if you’ve established early in the campaign that magic is a natural force, not unlike the weather, players would rightly protest if you later declare that the council of mages has shut down wizardly access to magic, if that was a storyline you were planning to run.
Overall, it is rarely a bad thing to provide details, even of the more obscure aspects of the setting. Just know what is worth spending time on, and be careful not to paint yourself into a corner!