This is your "inventory" bag. Click on your ship, then in your menu click Cargo Hold and "Show Content" and you can see what and amount of resources you possess. You can also split one stack into two by hovering your cursor over the stack to split and clicking "splits. Pressing J on your keyboard or clicking it on the side panel next to your inventory will kick out (the J stands for "Jettison") and it will be like discarding your resources. Others are able to take anything that has been jettisoned as they like.

You can also see how much someone else is carrying in their cargo hold via inspection. However which resources they are carrying will be unknown unless one devises a method to discern what material is onboard a different ship (this feature may become available in the future).