This is the 16743rd most frequent Russian word.
слабую
"Слабую" means "weak" (feminine accusative singular form).
Here, "слабую" is used as an adjective (weak) describing the type or intensity of tea.
Я предпочитаю слабую чай за завтраком.
I prefer weak tea for breakfast.
In this case, "слабую" indicates the characteristic of being weaker or less concerning an argument's position.
Он всегда выбирает слабую сторону в споре.
He always chooses the weak side in a debate.
"Слабую" in this sentence modifies "лампочку" (bulb), referring to its dim or lower intensity.