Puberty occurs in two separate stages, genital (penis and testicle)
development and pubic hair
development. We number each of these
stages 1-5, 1 being child and 5 being adult. That means 2-3-4 are the stages of puberty. Depending on your development, a number is assigned to genital development (G) and a number is assigned to pubic hair development (P).
For example, if you have just started growing hair you might be stage P2. It's possible to be P2 and still be G1.