Artifacts are elements that provide additional information on a process with the purpose of making it more self-contained. They don't have an effect on the behavior of the flow while it is being executed.
There are two types of artifacts:
Group: It allows you to conceptually group the elements of a diagram. Dragging elements to a group associates them with it, and if later the group is deleted, all its associated elements will be deleted as well. Likewise, if the diagram is moved, all the elements will be moved at the same time.
Text Annotation: It allows you to add text to a process in the form of a tag. These tags can be connected to the process elements as part of a conceptual association which doesn't affect the flow's behavior.
Below are examples of each one of these symbols:
Group Text Annotation