It's a nitrogenous base in Nucleic Acids. In DNA, the nitrogen in the bottom left is bonded to a third carbon instead of a hydrogen. The third carbon is part of a deoxyribose 'molecule'. In RNA it's part of a ribose molecule. Those sugars are then bonded to phosphate groups. Cytosine is hydrogen bonded to guanine through 3 hydrogen bonds so a C-G bond is stronger than an A-T bond where there are only 2 hydrogen bonds.
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