Stretch goals are not part of the official Kickstarter platform. Rather, a stretch goal is a funding target set by the project creator beyond the original Kickstarter goal. Stretch goals as a term and a practice emerged from the Kickstarter community as a way for creators to "stretch" beyond the initial, official goal of the Kickstarter project and raise more money (and often make cooler stuff!). Funds are collected whether stretch goals are met or not, as long as the project has met its Kickstarter funding goal.
What happens when a project is overfunded varies depending on the project, and stretch goals are not right for every project. If you are thinking about stretch goals you should consider:
Project complexity: Stretch goals can make a project more difficult to complete. Fulfilling rewards can be demanding, and any time you add new stuff (more songs on your album, better materials or options for your creation) the demands increase. More costs to consider, more things to ship — things can get complicated fast. It can be tempting to add stretch goals — especially if a project quickly exceeds its initial funding goal — but this shouldn't be done without careful planning.
Communication: If adding stretch goals, think carefully about how to announce them to your community of backers. Take the time to explain your intentions, your motivations, and your plans. Any changes made midstream should be accompanied by an assurance that you are honoring your initial promises to backers. Simply proclaiming, “New goal!” without recognizing what you’ve achieved together can rub backers the wrong way.