Question logic allows you to crate path for your respondents. For example if respondent selects answer choice A he will be see page 3, if he selects B he will see page 4, if he selects both A and B he will see pages 3 and 4.
To add question logic :
Multiple choice question logic hides pages for which you set logic and shows them if respondent selects answer choice.
NOTE. You have to divide your survey into pages before you start to design question logic.