From this page of the Baldur's Gate 3 guide you will learn how to throw objects. Moreover, here you'll find information what pre-requisites are there to item throwing.
As you explore your surroundings, you can pick up many different objects. The item lands in the backpack of a hero who performed the action.
This feature comes in handy at certain moments. For example, you can hit your opponent with an object. In addition, if there's a gas trap in the area that you can't disarm, you can always cover it with an object that is large enough.
These objects can then be thrown. To do this, simply select the "throw" option. Then a special menu will appear where you will see a list of items you can throw at your opponent or trap. This action costs one action point.
Once you have decided on the object, simply click on it. Now, an area will appear on the screen that will tell you how far you can throw the object. Now all you have to do is to choose a target.
How big an object you can throw depends on the strength of your character - the more points you have assigned to that statistic, the larger the objects you can throw.