When we factorise an expression, we are looking for simple factors that multiply to get the original expression. Usually it is very natural to factorise something like a quadratic in x. For example:
x^2 + 3x + 2 = (x+1)(x+2)
But there are other situations where factorisation can be applied. Take this quadratic:
x^2 - 9x = x(x-9)
This second example is closer to the question in hand. Just like x was a common factor to both x^2 and -9x, we are looking for a common factor to both 6b and 24bc. The common factor is 6b.
Hence 6b + 24bc = 6b(1 + 4c).
I hope this helps you :)