Freestanding Elements are thin elements in your model that are exposed on 4 or more sides.
Why is it Relevant? Similar to small overhangs, freestanding elements like pipes, arms, legs, posts, beams, airplane wings etc. are susceptible to breakage if they are too thin for the printing and post processing of the model.
For additional information on why a part may break, see the Small Overhangs post. In that post, large overhangs are mentioned. If the portion of your model in question is large compared its connection and support on the model, you will want to use the minimum thickness specified for Freestanding elements at the connection point and throughout the piece.