Full explanation
A pivot is a major change in your product, your audience, or your business model. It happens when you realize your initial idea will not work and you decide to try something different with what you have learned.
Pivots are normal. Most successful startups pivoted at least once. The skill is to recognize early that something is broken and to act, instead of pushing on a dying idea.
A good pivot keeps what works (your team, your knowledge, parts of the product) and changes what does not.
Concrete example
You built a CRM for restaurants. Restaurants do not buy. But you notice food trucks love your tool. You pivot and rebuild the product for food trucks only.