What does this symbol mean?

If you're talking about this symbol: Φ, it is the capital Phi. A letter of the greek alphabet. It represents various things in mathematics and philosophy, including the Golden Ratio, which is if the ratio between the sum of two quantities and the larger one is the same as the ratio between the larger and small quantities. The ratio itself is approximately 1.618. Certain artists (Da Vinci and Mondrian, for example) and architects have tried to incorporate the ratio into their works.