Both are real wood. The difference is in the construction, not the quality.
Solid wood is machined from a single piece of timber, typically 18mm to 22mm thick, the same species all the way through. Oak, walnut, ash and so on.
Engineered wood has a genuine hardwood top layer bonded onto a core of cross-bonded plywood. The surface is real oak or real walnut, identical to look at once it's down. The engineering is underneath.
That layered core is the whole point. It cross-braces itself, so the board moves far less with changes in temperature and humidity than a solid plank does. In a British house, with damp winters outside and central heating drying the air inside, that stability matters.
A note worth making plainly: engineered wood is not laminate. Laminate has a photographic print on a fibreboard core. Engineered wood has real timber on top. People conflate the two constantly and it isn't a fair comparison.