This is the 710th most frequent Japanese word.
る
"る" is a Japanese hiragana character, often used as a verb-ending in dictionary (plain) form for ichidan (group 2) verbs or as part of words.
Here, 'る' is the ending of the verb in its continuous form, indicating an ongoing action.
猫がベッドで寝ている。
The cat is sleeping on the bed.
In this case, 'る' is part of the dictionary form of the verb, representing a habitual action.
彼は毎朝早く起きる。
He wakes up early every morning.
In this sentence, 'ならない' is part of a grammar structure indicating obligation, and 'る' is part of the dictionary form of the verb indicating a state of necessity.