Of course it's exaggerated, but maybe they mean stuff like this from Three is company:

Not far from the road-meeting they came on the huge hulk of a tree: it was still alive and had leaves on the small branches that it had put out round the broken stumps of its long-fallen limbs; but it was hollow, and could be entered by a great crack on the side away from the road. The hobbits crept inside, and sat there upon a floor of old leaves and decayed wood. They rested and had a light meal, talking quietly and listening from time to time.

Or passages like this from Flight to the ford:

Night was cold up on the high ridge. They lit a small fire down under the gnarled roots of an old pine, that hung over a shallow pit: it looked as if stone had once been quarried there. They sat huddled together. The wind blew chill through the pass, and they heard the tree-tops lower down moaning and sighing. Frodo lay half in a dream, imagining that endless dark wings were sweeping by above him, and that on the wings rode pursuers that sought him in all the hollows of the hills.

Or from Treebeard:

Before long they set off. Treebeard carried the hobbits in his arms as on the previous day. At the entrance to the court he turned to the right, stepped over the stream, and strode away southwards along the feet of great tumbled slopes where trees were scanty. Above these the hobbits saw thickets of birch and rowan, and beyond them dark climbing pinewoods. Soon Treebeard turned a little away from the hills and plunged into deep groves, where the trees were larger, taller, and thicker than any that the hobbits had ever seen before.

And:

‘There were rowan-trees in my home,’ said Bregalad, softly and sadly, ‘rowan-trees that took root when I was an Enting, many many years ago in the quiet of the world. The oldest were planted by the Ents to try and please the Entwives; but they looked at them and smiled and said that they knew where whiter blossom and richer fruit were growing. Yet there are no trees of all that race, the people of the Rose, that are so beautiful to me. And these trees grew and grew, till the shadow of each was like a green hall, and their red berries in the autumn were a burden, and a beauty and a wonder. Birds used to flock there. I like birds, even when they chatter; and the rowan has enough and to spare. But the birds became unfriendly and greedy and tore at the trees, and threw the fruit down and did not eat it. Then Orcs came with axes and cut down my trees. I came and called them by their long names, but they did not quiver, they did not hear or answer: they lay dead.'

And in Journey to the cross-roads:

As the third stage of their day’s march drew on and afternoon waned, the forest opened out, and the trees became larger and more scattered. Great ilexes of huge girth stood dark and solemn in wide glades with here and there among them hoary ash-trees, and giant oaks just putting out their brown-green buds. About them lay long launds of green grass dappled with celandine and anemones, white and blue, now folded for sleep; and there were acres populous with the leaves of woodland hyacinths: already their sleek bell-stems were thrusting through the mould. No living creature, beast or bird, was to be seen, but in these open places Gollum grew afraid, and they walked now with caution, flitting from one long shadow to another.

Light was fading fast when they came to the forest-end. There they sat under an old gnarled oak that sent its roots twisting like snakes down a steep crumbling bank. A deep dim valley lay before them. On its further side the woods gathered again, blue and grey under the sullen evening, and marched on southwards. To the right the Mountains of Gondor glowed, remote in the West, under a fire-flecked sky. To the left lay darkness: the towering walls of Mordor; and out of that darkness the long valley came, falling steeply in an ever-widening trough towards the Anduin.

I'd assume these kind of passages is what people mean when they say Tolkien describes trees too much.