Shall Issue means the permit issueing authority of the state 'shall issue' a CHP upon application if the applicant is not otherwise disqualified.
Compared to 'May Issue' meaning the government authority 'may' at their discretion issue a permit.
If you apply in Maryland, a may issue state, you must supply them with a, in their eyes, good a valid reason for 'needing' a permit. In shall issue states you do not need a reason.
In a nutshell a shall issue state must issue a permit if you are not otherwise disqualified. In may issue states it is at their discretion. Some shall issue states are also call Right To Carry states.