Question about MCB(Miniature Circuit Breaker)?

I just started learning about MCB's in school.At the momment I am VERY confused about it,whats the relation between In,Ib and the tripping factor? There's also a graph on the different types of MCB's (1,2,3,4,A,B,C,D).Also,I have no idea how to read this graph :

https://www.tlc-direct.co.uk/Figures/3.18b.gif

Thanks

You didn't show the graph labels, so I can only guess. The x-axis is apparently current and the y-axis is time. From that you can see how long it takes a particular circuit breaker to trip at a particular overload current.

Thus, for example, the 6A breaker will trip in about .02s at 37A but will take about 30s to trip at 20A, and about 800s to trip at 10A.

In general the more over-current, the shorter the time to trip for any breaker. The idea is that the time it takes for an overload to damage the circuits depends upon the magnitude of the overload and the circuit breaker delay to tripping is designed for that.

While crutschow didn't mention it, you should be aware that there are two types of breakers, thermal and magnetic, each of which behave differently.