[R] What does the : operator mean in glm formulas

Hi,

I see the following is the credit scoreing in R guide :

m2<-glm(formula = good_bad ~ checking + duration + history+ purpose +amount + savings + employed + installp + marital +
coapp +age + other + depends + telephon + foreign +checking:amount

What does checking:amount mean?

Regards,

Xiaobo Gu