。
The tree spread its branches wide.
Literal
Tree [topic-は] branch [object-を] widely spread.
広く is the adverbial form of the i-adjective 広い ('wide, broad') — i-adjectives form adverbs by changing い to く. Attaches naturally before the verb. The transitive 張る ('to stretch, to spread') takes 枝 as its object, with the tree as the agent.