Learn Spanish (Spain): El pollo está en el horno. Ahora se lo saco. - The chicken is in the oven. I take it out for them now.

You are exactly right that 'for them' is normally les. However, Spanish has a rule: when the indirect object pronoun (le or les) comes right before the direct object pronoun (lo, la, los, or las), it changes to se. Saying 'les lo' is hard to pronounce smoothly, so les lo saco becomes se lo saco.

What exactly does the lo refer to here?

The lo means 'it' and refers back to el pollo (the chicken). Because pollo is a masculine, singular noun, we must use the masculine singular pronoun lo to replace it.

Could se lo saco also mean 'I take it out for him'?

Yes, it absolutely could! Because both le (for him/her/you formal) and les (for them/you all) mutate into se before lo, se lo saco is completely ambiguous on its own. It could mean 'for him', 'for her', 'for you', or 'for them'. In real life, the context of the conversation makes it clear who you are cooking for.

Why use the present tense saco instead of a future tense like 'I will take'?