What is a resonance structure?
A resonance structure has delocalized electrons
Resonance structures have two or more possible electron structures. Often, they are showed with dotted lines or two+ structures with an arrow between them.
The electrons distribute themselves evenly between both configurations (probability-wise) so that what we end up is roughly an "average".
This is a good example of resonance.
Here is the unhelpful technical definition: "The idea of resonance is that the actual molecule is a definite structure that is a hybrid of two theoretical structures"