You can set the probability that a particular question in your survey will be shown to a respondent. This means that if you create a question and set the probability to 50%, there is a 50% chance of that particular question being shown to a given respondent. In short, 50% of respondents will see the question, and 50% will not.
This is useful if you have a high number of questions and need a representative sample for each one, instead of showing a long survey, you can get a sample of responses on certain questions.
To do this: In the survey creator, click the 'advanced settings' option at the top of the question window. Then simply drag the probability slider in the upper left side of the question window. Note that the probability can be changed for each question (with a default probability of 100%).