Is it sufficient when saying "Alhamdulillah Rabb Al-Alameen" to feel praise for Allah without recalling the meaning of each word, and is knowing the meaning of Athkar and supplications obligatory?

The reciter of the Qur’an must recite with contemplation, because it is the intended and desired outcome through which hearts are gladdened and illuminated. The Prophet, peace and blessings be upon him, condemned those who recite the Qur’an without contemplation.

As for supplications and ذكر (remembrances), it is better to contemplate their meanings. A Muslim receives reward merely by uttering them, even if he is heedless of their meaning. However, the most complete is to have presence of heart and gather one’s intention with the remembrance.