Can someone please explain to me in plain simple English 'exactly' how the Saved Toolbars work?
Saved toolbars can be used to store your current hotbar (all 9 items) in one of the 9 available slots. In order to save (store) your hotbar you need to press the saved key item + a number: 1 to 9. By default the keys to use are C to save and X to load, these can be customized in your control settings under the 'Creative mode' section. So if I want to save my current hotbar to position one I'd press: C + 1 (press C, then while keeping this pressed press the numerical 1 key). If I would like to empty my hotbar (apart from using shift-click on the destroy item option in your survival inventory slot) I could load an empty toolbar. Since I only saved 1 I can press: X + 2 and it would load an empty toolbar, effectively clearing out my hotbar but not my main inventory.
Note: With hotbar I'm referring to the bottom row in your inventory, the items you can activate by pressing 1 - 9. With toolbar I'm referring to one of the 9 saved toolbars (in the Saved Toolbars tab).
This feature is useful because you can quickly switch between tools. Or even collection of tools (I keep a selection of shulker boxes on the 9th toolbar, so when I need a custom item (for example) I simply load the 9th toolbar (x+9, but in my example that would be control+9), place my custom items shulkerbox (renamed cyan shulker box) and get what I need).
Another example: lets say I'm working on a redstone project. I have: redstone dust, a redstone torch, repeater, comparator, observer and a sandstone block in my hotbar. However, I'm also working on a bigger project where I'm using a shovel to make a road, signs to... you know and I'm also carrying a torch, a command block, structure block and a crafting table. All in all I'm using 12 different items and those won't fit on 1 hotbar.
So I saved my redstone tools to my first toolbar and my "project tools" to the second. So whenever I'm working on redstone I press control-1 to load my first toolbar (as mentioned earlier: I changed the hotkey from x to control), but whenever I need to expand a bit on my project I press control-2 and have all those tools available.
Maybe also interesting to know: your toolbars are saved locally (in the file hotbar.nbt). Yet this also means that your tools are carried across all places where you have access to Creative mode. Including servers. So if you have some items which you need to move from 1 single player world to the other then don't bother yourself with chests and structure blocks: Just place a shulker box, put your item(s) in there, pick it up and place it on your hotbar. Save said hotbar, close this world & open the other you wanted, and load the toolbar again. Presto: your shulker box with all your items will sit right in your hotbar, waiting for you to place it.