Items have a property called their Bulk, representing how heavy and unwieldy they are to carry, whether in your hands or in a bag or pouch.
Items with Bulk L are small and light enough that they don’t add too much weight on their own. Every 10 items with Bulk L you are carrying adds +1 Bulk to your load.
Your Carrying Capacity — how much Bulk you can carry without taking any penalties — is equal to your Strength score (the one that is increased by Attribute Boosts, not your STR modifier). Some Feats, Keystones, and items can increase this number.
If you carry Bulk that exceeds your Carrying Capacity, you gain +1 level of the Slowed condition, reducing the number of actions you can take on your turn and halving your speed while traveling.
If you carry more than twice your Carrying Capacity in Bulk, you gain +2 levels of the Slowed condition.
You gain +3 levels of Slowed while carrying more than three times your Carrying Capacity in Bulk, and cannot leave your location until you are no longer so overburdened.
For reference, in case a character needs to be carried out of danger while unconscious, most flesh-and-blood Medium creatures range from 8 (small humans) to 12 (large Jotuns or Minotaurs) Bulk when carried, and Small creatures from 4 to 6 Bulk.