Many items in Baldur's Gate 3 can be stolen from their owner. This includes free-standing objects in the environment that can simply be picked up, and objects held by creatures that can be stolen via the pick-pocket mechanic. This page tries to explain all mechanics involved in stealing.
The explanations below are based on in-game experimentation, and may be inaccurate or incomplete.
Picking Pockets
When you're currently Hiding and not seen by a creature, you have the option to start pick-pocketing from it. In fact, this is the default action when clicking on another creature while hiding. Starting the pick-pocket process opens a dedicated UI for the mechanic.
In the pick-pocketing window, you can choose any one item that the creature holds to target it for stealing. The window will show you the Difficulty Class of stealing that particular item, which seems to depend on its value and/or its weight. You can then confirm the attempt to roll the D20.
An unsuccessful attempt means that the creature immediately notices and accuses you of stealing. It leads to the typical "you've been caught stealing" conversation.
However, a successful roll on the pick-pocket action doesn't guarantee that you will get away with it either. The creature will notice that it's been stolen from a while after, and start investigating. This is visually indicated by an overhead text such as "I've been robbed!" The creature will move around a little, and if during this period it sees you or anyone else from your party, you will be accused of the crime.
It seems that being accused during the investigation period is not equivalent to being caught immediately, and allows you to convince the creature of your innocence without any further negative consequences.
If your party stays far away enough during the investigation period, the creature will end the investigation and go back to normal. After this point, it seems that you're fully safe from negative consequences, even if you walk right up to the creature wearing the item you've stolen from it and talk to them.
Stealing Free-Standing Items
Items in the environment which belong to another creature are indicated with a red outline and text. They can be stolen without consequences if you're careful.
To steal such an item, you first need to make sure that you won't be noticed in the moment you pick it up. This means not only being outside the field of view of the owner and their allies, but also not being heard. If you're Hiding, you won't be heard while picking it up.
What happens after successfully picking up the item that doesn't belong to you, seems similar to what happens after a successful pick-pocketing roll: Tthe creature notices a moment later that it was stolen from, and begins investigating. You and your party must stay away during the investigation period to make sure that you won't be accused. Once the investigation period is over, and the creature you stole from goes back to normal, you're safe.