If you have multiple topics in your questionnaire and would like to separate them, then use the 'Label' question type. The question should be inserted before each new topic segment, i.e. before the first question of the new topic. In the question box, you can type the title of the topic segment, which can also be bolded and enlarged, or you can type an opening address, which can also be formatted however you like.
You can also visually separate the title or the label from the rest of the questions by clicking on the 'Label' question toolbar to open the question settings, and selecting 'Yes' under the 'Line after label' option.