Meaning: This sentence describes where the sound of birds singing can be heard. It's like saying,
Choice of Words: "Birdsong" is the sound of birds singing. "Echoes" means to be repeated as sound. "Through" means from one side to the other. "Forest" is a large area of land covered with trees. "Trees" are tall plants with a trunk and branches. When you put it together, it means the sound of birds singing travels through the trees in the forest.