House flipping is when a property investor buys a distressed property, fixes it up, and then sells it again. A distressed property is one that was either foreclosed on or has significant repairs and fixes needed. House flipping is sometimes called a "Rehab".
Property investors buy distressed properties because it often sells at a fairly large discount to what the property could be worth. Once the house is fixed up into livable condition, the home value appreciates significantly. Most house flippers are short term holders and may use non-mortgage financing options like hard money.