Using traditional terminology, absolutes are prepared from concretes.
Flowers will be extracted with a hydrocarbon solvent such as pentane and the pentane evaporated to produce the concrete. A concrete will typically contain large amounts of plant waxes that will not easily dissolve in alcohol.
The concrete will be extracted with alcohol and the alcohol evaporated to produce the absolute which now contains much less plant waxes and is more suitable for use in alcohol based perfumes.
Absolutes are more expensive but also more concentrated. Concretes can be used in solid perfumes but for alcohol based perfumes you will need absolutes.
Some places will also sell the wax that is left behind when a concrete is extracted with alcohol. These waxes still contain amounts of fragrant materials and can be used in solid fragrances.
More recently there are now materials produced using liquified carbon dioxide as the extracting solvent. This produces a material somewhere between an absolute and a concrete.