Refers to a pine tree, often specifically the Japanese red pine (Pinus densiflora) or black pine (Pinus thunbergii) in Japanese contexts.
庭に松の木を植えた。
I planted a pine tree in the garden.
Compound of 松 (matsu, 'pine') + の (genitive particle) + 木 (ki, 'tree'). The reading まつのき is a straightforward combination.