A solid hardwood flooring quite simply is a flooring made from many numbers of different types of tree or species of tree but is manufactured from one single piece of wood.
The alternative is an engineered board and the key difference to the engineered board will have a top layer of hardwood with a manmade back. The hardwood flooring will normally be tongued and grooved this will appear on all four sides of the board which means they will link together and you can install them without seeing any gaps.
Solid wood floors are extremely strong and durable because of the large amounts of wood that sits above the tongue that maybe sand many times. The recommended fitting for these types of boards are a fix or permanent fix to the sub floor. This would mean either fully gluing the board to your sub floor whether it is concrete or sheet material timber or secret nailing at an angle through the tongues to fix to the sub floor. This method would normally be used if you are fixing to floor boards or to timber sheet materials.