This is the 16969th most frequent Russian word.
сосредоточьтесь
"Focus" or "Concentrate."
In this sentence, 'сосредоточьтесь' is used as an imperative verb encouraging the listener to focus their attention on the lesson.
Сосредоточьтесь на уроке, чтобы лучше понять новую тему.
Focus on the lesson to better understand the new topic.
Here, 'сосредоточиться' is used as an infinitive verb expressing the action of concentrating or focusing.
Я просил его сосредоточиться во время игры, но он все равно отвлекался.
I asked him to concentrate during the game, but he kept getting distracted.
In this example, 'сосредотачиваться' is used as a reflexive verb in present tense, showing the continuous action of focusing or concentrating.