Fluorenone is an aromatic compound that contains a five-membered ring with a carbonyl group attached and two benzene rings fused on either side.
'Aromatic' means that the compound contains an alternating network of double and single bonds all the way around each benzene ring. the five-membered ring that looks like a pentagon is sandwiched right in the middle of each benzene ring. These types of ring systems that are joined together like that are called fused ring systems. the five-membered ring has a benzene ring 'fused' to each side of it.
A carbonyl group in organic chemistry is always defined as a carbon-oxygen double bond. If the carbonyl group is bonded to two other carbon atoms (as with fluorenone), than that particular functional group is known as a ketone.