A tree-lined road, street, or path has trees standing along both sides of it, in a row. That is the whole meaning — but the word does more work than that. It almost never appears as a neutral fact. It appears when someone wants you to feel that a place is pleasant: quiet, shaded, green, a bit expensive. Estate agents love it. Novels love it. 'A tree-lined avenue' tells you as much about the money and the mood of a neighbourhood as about the trees. Grammatically it sits in front of a noun and stays there: a tree-lined street, tree-lined boulevards. You would not normally say 'the street is tree-lined' — English speakers say 'the street is lined with trees' instead, which is the same picture as an ordinary sentence. Keep the hyphen; it is one of those two-word adjectives that needs it. And notice the trees must form a line. Three trees scattered in a park is not tree-lined. The word needs the row, the repeating shape you see when you look down the road.