There are two types of collectables in the game. Unlockables and Panties.
Items can be picked up, or given to the player by a character. The significance of these items will vary depending on what they are.
In Easy mode a message will be displayed to indicated something is availble to pick up
Unlockables, as the name suggests, will unlock something. This might be a smartphone background, gallery image or required item for a future event etc…
Panties are a special collectable that, like items, can be picked up or gifted. They are considered an in-game trophy and are displayed on the inside of your closet door.
Since the collectables exist in the game world, there is a chance of other characters viewing these items.