How is a policy expressed?
I wanted to know what is the answer to the question 'how is your policy expressed'?

Typically when this is asked, what the asker wants to know is something like:
How do you represent the action choice in the output?
For example, if there are N discrete action choices, and your policy is stochastic, you could say something like: &quot;The policy is expressed as a vector size N, with each value being the probability of choosing a matching action.&quot; You may also want to enumerate the actions - to say which index of the vector corresponds to which specific action. That depends on what need the person communicating with you has for understanding your policy.
Plus maybe:
How do you represent the state in the input?
The same thing here, give the state representation required to make the policy function work according to your implementation. Again, whether you should supply this depends on why the person asking you wants to know.

Am I supposed to give an analytical function?

With the right level of detail, that should be fine. The input and output domains plus maybe the index map (of which input and output features are in which positions). Note that because you are asked for how the policy is expressed, you would usually stop at the point that you have defined the probability distribution over the action space.