What is ansatz? Is it an abstract hypothetical concept, an actual circuit, or an algorithm?
An ansatz is a guess. In quantum computing our guess (or ansatz) is a circuit where we know the gates but not the parameters that go in those gates. For instance, I may have a problem where I need to model a sine function. I know that an RX and a ROT gate can give me a good model for my problem, so my ansatz will be a circuit with an RX and a ROT gate.
Once I have my ansatz I can optimize over the parameters (the angles in each gate) in order to get a circuit that effectively models my input-output function.
Here you can find a deeper explanation into what an ansatz is, but I hope this mini explanation gives you a good start.
Please let me know in case it’s not clear, or in case you have any further questions!