Learn Polish: Mój mąż to pracoholik, podczas gdy ja wolę odpoczywać. - My husband is a workaholic, whereas I prefer to rest.

It can mean both! Just like the English word "while", podczas gdy can show contrast (as it does here, meaning "whereas") or it can show two actions happening at the same time. The context of contrasting a workaholic with someone who likes to rest makes it clear we are using its contrastive meaning here.

Why is the pronoun ja used here? Doesn't Polish usually drop subject pronouns?

Polish does usually drop them, but you must include the pronoun when you are making a direct contrast. Because the sentence contrasts your husband (mój mąż) with yourself, you need ja to balance the sentence and emphasize that contrast.

Why does the sentence use to instead of jest for "is"?

Using to is a very common way to link two nouns in a basic definition. The great thing about using to is that the following noun stays in the basic nominative case (pracoholik). If you used the verb jest, you would have to change the noun to the instrumental case (Mój mąż jest pracoholikiem). Both are correct, but to is often more natural for simple labels.

Is the comma before podczas gdy strictly necessary?