A grammatical expression attached to the masu-stem of verbs to indicate that an action or state is in progress (i.e., 'is doing / becoming'); commonly used in written language.
町の景色は少しずつ変わりつつある。
The town's scenery is gradually changing.
桜が咲きつつあり、公園は花見客でにぎわっている。
The cherry blossoms are beginning to bloom, and the park is bustling with hanami visitors.
古い建物が取り壊されつつあり、新しいビルが建っている。
Old buildings are being torn down and new ones are being built.