What does the comma mean in a card effect's cost?
It's a single cost and you pay both parts. From the Comprehensive Rules:

602.1 Activated abilities have a cost and an effect. They are written as “[Cost]: [Effect.] [Activation instructions (if any).]”
602.1a The activation cost is everything before the colon (:). An ability’s activation cost must be paid by the player who is activating it.
Example: The activation cost of an ability that reads “{2}, {T}: You gain 1 life” is two mana of any type plus tapping the permanent that has the ability.