This is the 931st most frequent Russian word.
плохо
"Bad" or "poorly."
"Плохо" is an adverb that describes the manner of performing the action "пою" (sing).
Я плохо пою.
I sing poorly.
"Плохая" is an adjective form agreeing with "погода" (weather) in gender, number, and case.
Сегодня плохая погода.
Today the weather is bad.
"Плохо" describes a state or condition, explaining how the speaker feels (similar to "unwell").