In short, clear the Cached data in the Storage menu.
Open the Settings app on your phone, and locate the Storage menu.
Wait a bit for the values to be calculated, and have a look at the Cached data – if it’s too large, you can easily free up some space by clearing it.
Tap on Cached data entry, and accept to clear it – as it is only cache data, meaning temporary data used by applications, you can easily get rid of it with limited risk.
See the difference, it should now be much lighter.
Another step you can do, in the Storage menu, scroll down to the Misc. entry, which might be quite high as well.
Go in, and have a deep look at all the entries. You might have some data remaining on your phone from apps you don’t use anymore or have uninstalled a long time ago.
Select the entries that you don’t need anymore, and tap the trash on the top right.
Check out your phone storage, it should now look much happier !