itch.io

Hm, it's possible the game this is based on, Knave (CC BY Ben Milton) explains it more clearly:

PCs have a number of item slots equal to their Constitution defense, and items they carry must fit into available slots. Most items take up one slot, but some take up more. Some small items can be bundled together into a single slot. Ask the referee if you are unsure.

So, as an example: 

Let's say you make a Wizard, which starts with the lowest possible carrying capacity — 11 item slots. 

(Carrying capacity is based on your Constitution score; the Wizard has a +1 CON bonus, which means a +11 CON defense — it's just the bonus plus ten — which determines how many item slots.)

The Wizard has an academy robe, which takes up 1 slot, and a silver-pommel cane, which takes up another 1 slot. Their spells live in their mind, and so take up no item slots. (This is different from how spells work in Knave.) As such, the wizard is carrying 2 slots worth of items to start. This means they have 9 slots free — they can pick up 9 more items before it starts to penalize them. 

In addition, for each unused item slot a character has, they have 1 point of stamina. Stamina is used to cast spells and pull off stunts in combat. If you're a Wizard, this is a major incentive to not carry around lots of heavy weapons and armor; traveling light means being able to cast more spells. (This is another difference from Knave; there's not much reason to leave slots unused in that game, except to avoid erasing one thing to write in another every time you find a new item you want to carry around.)

(As a side note, it occurs to me that it may be confusing you — and possibly other readers — that this game and Knave use same term, "slot," for both inventory capacity and item weight. It might help to make a distinction between slots as carrying capacity, and weight as how many slots an item takes up – as in, a dagger with 1 weight takes up 1 slot in your inventory.)

I hope this helps!