Merging surveys means that the questions from both surveys will be merged into one larger survey. The original surveys will no longer exist. If you want to keep the original surveys you should duplicate them first before merging.
Merging surveys can allow you to analyze data collected in two similar but seperate surveys, or from two identical surveys sent to two different groups for example.
You choose which of the two surveys is to be the Primary Questionnaire and which is the one To Be Merged. The respondents and questions from the survey being merged will be added to the Primary questionnaire thereupon the to be merged survey will be deleted and thus respondents lost.