I don't understand the meaning of the QoS parameter in the mqtt in node. Is it some kind of filter?
This is generally considered the best explanation: What is MQTT Quality of Service (QoS) 0,1, & 2? – MQTT Essentials: Part 6 (hivemq.com).
I'm not asking what the QoS values mean, I understand what they mean when publishing messages.
I'm asking what the purpose of setting a QoS is when receiving messages.
Oooh, that's the QoS of the subscription, right?
Yes, QoS levels of publishers and subscribers can be different, which could have effects on message delivery.
For reference, search for the paragraph "Downgrade of QoS" in the link provided above.
The subscription QoS will be ignored if it's higher than one published to the broker according to spec
If it's lower than the published one , then it will be obeyed
e.g pub as 0, sub as 2 - broker will use 0 - pub as 2, sub as 1, broker will use 1
But the mosquito broker can be (mis) configured to not do this