A Specification tells you the important features of an item you are purchasing. These will vary depending on the item.
For buying raw materials and chemicals, a specification includes the ranges of chemistry, the density of the material, color, and any limits on impurities such as heavy metals.
For finished products, the specification includes measurements such as weight, length and geometry features, and any functions performed by the item.
Some examples of what you may see on a spec sheet:
Glass Dropper Bottle: Blue virigin glass. Dropper with bulb. Dropper holds 1mL liquid. Bottle holds 15mL. Total weight 0.5 oz.
Ground Fine Pumice: 1 lb package. Particle size 149 to 240 um particles.