Most people are familiar with small molecule medicines, like aspirin or oral antibiotics. These are produced via a series of chemical reactions, and their exact structure is simple and easily identified.
In contrast, biologic medicines are very large, and very complex. Biologic medicines are made in living cells, grown in a laboratory. By way of comparison, if you think of a small molecule as a small house, a biologic medicine could be compared to the Empire State Building, in relative size and complexity.