Encumbered
Moving becomes harder when you are carrying too many items in your inventory. Carry less or unlock more slots with a perk or other means.
The number is the number of items that are encumbering you.
Cause and consequences
Using encumbering backpack slots slows your character. Occupied space matters, not just the name of the item.
How to treat or respond
Store some items, combine stacks or increase capacity with suitable perks and equipment.
Prevention
Leave room for loot before a trip and keep a feasible return plan.