。
Wooden houses burn easily.
Literal
Wood-built houses [topic-は] easily burn.
簡単に ('easily,' the adverbial form of the na-adjective 簡単) modifies 燃える directly. This is one of the two main Japanese strategies for 'easily X-ed': add an adverb of ease, or attach ~やすい to the verb stem (燃えやすい). Both produce essentially the same meaning.