What is happening with my iPhone storage?

I really never understand the iphone storage.
It says it can't take any more photos because storage is full but the next day i can somehow take photos again which i find to be extremely strange, why is that occurring and has this happened to anyone else? Is storage automatically freeing up itself(this sounds highly unlikely and strange)

Thanks in advance if anyone knows why this is happening! And also how can my storage even be full when i have around 3 gb left?

You don't have 3GB left. It's used for system files.

You need to reset your phone to get rid of leftover garbage files. It won't erase anything else. Hold down your on/off (sleep/wake) button and Home button at the same time for about 6 seconds. Your phone will then perform a reset, which will take about a minute.