This is the 3351st most frequent Russian word.
поднимать
To lift; to raise.
In this context, "поднимать" is used to express the physical action of lifting or raising something, specifically a hand here.
Я поднимаю руку, чтобы задать вопрос.
I raise my hand to ask a question.
Here, "поднимать" is used figuratively to mean improving or elevating an abstract state, like the mood.
Он поднимает настроение своим юмором.
He lifts the mood with his humor.
In this example, "поднимать" conveys the introduction or bringing up of a topic for discussion.