This is the 14935th most frequent Russian word.
поджигать
To set on fire; to ignite deliberately.
Here 'поджигать', in past tense as 'поджёг', is used to describe starting a fire for a practical purpose.
Он поджёг костёр, чтобы мы согрелись.
He lit the campfire so we could warm ourselves.
In this sentence, 'поджигать' is used in its neutral sense, meaning 'to light something on fire'.
Я люблю поджигать свечи вечером.
I like to light candles in the evening.
In this context, 'поджигать' signifies a deliberate act of arson, which is illegal and carries negative connotations.