This video is from Qiskit.
So what is an Ansatz?
An ansatz is an educated guess about the value or form of an unknown function and is used to help derive the real solution of an equation
In quantum computing ansatz is usually a parameterized circuit often used in variational algorithms such as VQE.
This ansatz is used as a starting point or trial state which is then iteratively updated as more information is calculated.
Picking a good ansatz is important when running quantum experiments as a well chosen ansatz can drastically improve the accuracy of your results.