>What does “freedom to act” mean?

Just use the normal definition used in society. I generally like to refer to the legal system.

In a legal contract there might be certain conditions that restrict your freedom to act in certain ways.

It's about the "external" world influences on what we call a "person". So to what extent does the external world influence and control what the person can do.

So treat a "person" as a black box, that includes everything that goes into making up that person, so their DNA and all past environmental inputs that you would consider making a person what they are.

Then since that person is a black box, you can't know how they work. In such a situation would knowing the current environmental inputs be able to predict what that person does.

So lets use a real life situation.

You may offer that person the opportunity to commit to traffic drugs. In the normal case you can't completely know whether the person would traffic drugs, that person has the freedom to choose. (The fact they choose deterministically is irrelevant).

In another situation you threaten the person to kill the person's family if they don't traffic drugs. In this situation the external environment is limiting the freedom to act of the person. That person is going to very likely to traffic drugs. (The fact they choose deterministically is irrelevant).

There is a real difference between being coerced into committing a crime and not. The difference according to most/all court and justice system, most lay people and most professional philosophers is know as "free will". The only group that might not agree are amateur philosophers.

>It is a principle of fundamental justice that only voluntary conduct – behaviour that is the product of a free will and controlled body, unhindered by external constraints – should attract the penalty and stigma of criminal liability.
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>https://scc-csc.lexum.com/scc-csc/scc-csc/en/item/1861/index.do

In the case of R. v. Ruzic

>The accused had been coerced by an individual in Colombia to smuggle cocaine into the United States. He was told that if he did not comply, his wife and child in Colombia would be harmed.

The Supreme Court found that he didn't smuggle the cocaine of his own free will. He didn't do it in line with his desires free from external coercion. Hence they were found innocent.