Android has internal memory reserved for system and user blocks, that are separate from each other.
The warning you are getting indicates that one, the other, or the combination of the two are very close to full.
If you have apps you've installed that you don't use, uninstall them. If you've got things like photos by the thousands on the device, upload them to your computer and delete them from the device.
Most people are pretty lax about "housekeeping" on smartphones, and if you've got an older one that has limited ROM and RAM it's very, very easy to fill it up. I had an LG Tribute (LS660P) that could never even download and install an updated version of Android, even when reset and empty, because the amount of available memory was insufficient to do so.
You don't mention your make, model, and Android version - all of which are helpful to know when trying to answer questions like the one you're asking.