These are miscellaneous, consumable, limited-use random items you may wish to bring with you along your journey.
They could be anything from “Health Potions” (a beverage that instantly restores some Health) to “Galaxy Class Starship Key” (which lets you fly a faster-than-light starship around the galaxy). It could be practical things like rope, camping gear, and fishing poles. It could be fun things like a deck of cards, cell phone, or a notebook.
Browse through the inventory items database for some examples and ideas. It’s not an exhaustive or complete list by any means. Feel free to suggest your own inventory items; just be sure to get your Game Master’s approval. Some items cost money, others are free. Your Game Master can always override and change any price too. In fact, when first starting a game, your Game Master may let you pick any reasonable items for free.
You can only carry so many items at once, though.
The more skilled, educated, and talented you are (all six of your Attributes), the more items you can safely carry. If you have really good Aim or Evade, the better you are at holding and carrying things too. Increase any or all of those stats to increase your maximum inventory size.
Inventory items are definitely optional. You don’t necessarily need any. But you’ll probably find they come in handy, sooner or later, probably multiple times throughout your adventure.
During the story, if you find an item you really want to keep, but don’t have enough open inventory space for it, you’ll have to drop something else to make room — or ask one of your teammates to carry something for you.