This is the 7544th most frequent Chinese word.
记
"记" means "to remember," "record," or "note."
Here, 记 is used in the sense of 'recall' or 'remember', embedded in the compound verb 记得.
我记得今天是你妹妹的生日。
I remember that today is your sister's birthday.
In this sentence, 记 is used as 'note down' or 'record', shown in the verb phrase 记下.
请你记下我的电话号码。
Please write down my phone number.
Here, 记 implies 'to take note of', indicating the action of paying attention to and remembering details.