what is dihybrid cross?what is the raio of F2 hybrid?

When two characters or traits are studied during a cross, it is called dihybrid cross. 

For e.g. in a pea plant

• Yellow colour and round shape is dominant over green colour and wrinkled shape.

• Phenotypic ratio − 9:3:3:1
  Round yellow − 9
  Round green − 3
  Wrinkled yellow − 3
  Wrinkled green −1