What does “power” mean in “unequal resources and power”?

Here, power means the capacity to influence the terms of a relationship or to act effectively in pursuing one's interests.

In your labor context, it particularly means bargaining power.

For example:

A landowner can say:

“I will offer you these labor terms.”

A landless worker can say:

“I don't accept them.”

But if the worker has nowhere else to work and no land on which to support himself, his ability to reject the offer is limited.

The landowner therefore has greater bargaining power.

So:

Unequal resources = unequal possession of land, capital, savings, property, etc.

Unequal power = unequal ability to influence decisions, negotiate terms, or pursue one's interests.

They are related but not identical.