I have found very explained answer on quora.

Suppose there is a Simply Supported RCC beam subjected to udl(uniformly distributed load) throughout and this will produce sagging bending moment(often referred to as positive bending moment) over the entire span length.

i) Cracked Section :- Now due to such loading the compressive stresses will develop above neutral axis and the tensile stresses will set up below neutral axis of the cross-section. Now, we know concrete is very weak in tension and its tensile strength = compressive strength/10 generally. So as the load is being increased the magnitude of stresses will also increase and a time will come when the tensile stresses developed in concrete below N.A will be more than the tensile strength of concrete. So now concrete is not able to take up such tensile stresses as it gets cracked and the whole tensile stresses is now borne up by steel. So now below N.A the concrete area is found to be ineffective as it already got cracked and is unable to withstand the tensile stresses anymore. So, such a section where concrete gets cracked and is found to be in-effective below the neutral axis is called as Cracked Section.

Generally, the first tensile crack occurs at a stress value of 0.7root(fck) and this is also called modulus of rupture or flexural tensile strength of concrete(fcr). In other way we can also say that if the applied external moment M is more than the cracking moment Mcr, then also the section is said to be a cracked section.

ii) Un-Cracked Section :- As you already got an idea about cracked section, so uncracked section is such a section where the applied external moment(M) is less than cracking moment (Mcr) or you can say if the load is of such value that the magnitude of tensile stresses developed below N.A is less than the permissible flexural tensile strength of concrete(fcr) then easily concrete can take up such tensile stresses and the concrete area is also found to be effective below N.A. So such a section is referred to as uncracked section.