The practice of weighting data is very common in survey research. Typically, weighting is a post-data collection task that allows you to adjust your results so that they better reflect what is known about a population (the group about which you wish to draw conclusions).
As an example, if your population is 50% male and 50% female and, after collecting your responses you have more responses from females than males you'll need to adjust for this difference to better draw conclusions about your population. This means you will need to either reduce the influence that male responses have on your data or increase the influence that your female responses make.
At this time, Alchemer does not have an analysis tool for weighting data. For your weighting needs, we recommend using an advanced analysis tool (like SPSS) that allows you to create new variables post data collection.