Hydration?

I'm not completely new to sourdough baking, but after browsing a few sites about sourdough baking, I realize that I have a rather uncommon way of sourdough baking.

I think I made my first sourdough bread 20 years ago (but I'm not a very consistent baker... and I can buy excellent sourdough bread from a German organic baker here in the neighbourhood, so I'm not always tempted to started baking myself again.

My way of making a starter is very simple: start with a heaped tablespoon of flour and some water, and keep adding some flour and water twice a day.  (When I have an 'established' starter later, I only feed it once a day - I've never refrigerated it.)  (Here in Flanders we call sourdough 'desem', but we just prepare it like sourdough starter, not like Laurel Robertson describes a 'Flemish desem' in her books...)

I don't have a real recipe for my bread: I take 'some' flour (never really measured, weighed it), add some starter (never heard about a 'sponge', just added a bit of my starter to the dough), add water, knead...

When it seems too dry, I add a little more water, or more flour if it is too wet... Sometimes I add seeds, spices, nuts...

Oh yes, and at some moment in the proces I add a teaspoon of salt - at least, most of the time I add some salt, sometimes I do forget that...

Since (or at least I think it is 'since') I used only a small amount of sourdough, I have to wait quite some time before the lot has proofed.  (I often kneaded the dough before going to sleep, and baked it in the early morning, or made the dough in the morning, let it proof over the day and baked it after returning home...)

OK... long story to tell that I'm now completely lost... Although I have made some sourdough breads yet, I don't understand the recipes I find...
What is meant by a starter with a certain % of hydration?  100 % seems to me a starter that's just plain tap water, so that should be impossible.  100% hydration means 50% flour and 50% water? Volume % or weight %?