I'll see if I can explain this to you easily and show you a space example. Here is a storage breakdown to give you an example of how much it can hold.
I will base this example on 16GB iphone, if you want 32GB then simply just double the numbers to give you an idea of the storage used and how much you would have available.
Storage is broken down into 5 main things for this example, normally you can get into other things like cached data for apps or browser, books, messages, etc., but these are the main things.
1) iOS - The operating system obviously requires some space. On iphone, this is typically 4GB but could vary some in the years ahead as they make changes but this is what it is right now.
2) Photos - Lets just say 3meg per photo. Mine are more like 2 meg per photo, but I'm just rounding up as its better to over guess.
3) Videos - This is based on many things and can be HUGE if you make a long video. The really high quality video today means lots of space consumed. I just made a 2 second video roughly, and its 4 meg. So for this example, lets assume videos are 2 minutes so I'll round it to 250 meg for a video. Obviously this can vary alot.
4) Music - Between 5 meg and 15 meg per song, most of mine are around 7 meg so we'll use that.
5) Apps - My apps are typically between 2 meg on the low side, and 120 meg on the high side. I will just say 50 meg for an average for this example.
6) Free! - You cant fill it up completely, so lets say 1 GB is required for the iphone to continue being fast, it needs to some workable storage space.
So what does this mean?
On a 16 GB iphone, lets assume 5 GB consumed to start as listed above. You have 11 GB remaining to be used, so you could have the following stored:
Photos - 500 photos (1.5 GB used total)
Videos - 4 (1 GB used total)
Music - 500 songs (3.5 GB used total)
Apps - 100 apps (5 GB used total)
This should give you and idea, this is 11 GB used and leaves some space remaining for phone usage also. Obviously these numbers can vary, and you can copy your videos to computer if necessary, and copy photos to computer if you want to save them, then delete from the phone. That would easily recover 2.5 GB again in the example above. Not using any music in this example would free up 3.5 GB, so you can adjust these numbers to your own liking to get a good idea! :)