What is resonance structure and buffer system?
Resonance structure: A molecule or ion with such delocalized electrons is represented by several contributing structures is also called resonance structures or canonical forms.
Example: Resonance structures of CO2 are
Buffer system: A buffer is a chemical system that maintains a relatively constant pH even when strong acids or bases are added. Buffer solutions contain either a weak acid or a weak base and one of their salts. Buffers not only establish a pH, but provide ions to support conductivity. A buffer system is a chemical system consisting of a weak acid (in this case H2CO3) and a weak base (in this case bicarbonate ion = HCO3-).
Example: 
When the pH gets too high (high OH concentration), the OH- reacts with carbonic acid (H2CO3) to form HCO3- and H2O. When the pH gets too low (high H+ concentration), the H+ reacts with HCO3- to form H2CO3. Because H2CO3 is a weak acid, the H+ stays associated with the H2CO3. is a buffer system.