100% Full Power is a transformation-like power up that can be assumed by certain members of Frieza Race. It allows the user to draw out their maximum power and bulk out their muscles. Their defense and speed increases with this power up as well. However, it requires a tremendous amount of energy to simply maintain the form and is only used as a last resort, such as Frieza's fight with Super Saiyan Goku.
The form simply appears as a taller and much bulkier version the user's usually sleek and slim true form.
Not every member of Frieza's race possess the ability to go to 100% Full Power, with only Frieza and Frost being confirmed as users.
Naho Ooishi in an interview for Resurrection ‘F’ stated that Frieza's power has always been in a different league from the rest of his family. Thus it is possible that only a member of the race with an extremely powerful body can handle this level of power in their true form; like the mutant prodigy Frieza. On Namek, at full power Frieza's power level is 120,000,000; which is much higher than his true form's basic level of power.
Frieza utilizes his 100% Full Power state in order to counter Goku's newly attained Super Saiyan form, allowing him to achieve a level of power capable of matching Goku's for a few minutes.
During the Tournament of Power, it was revealed that Frieza still had the capacity of bulking himself up if he wanted, and also that Frost could take on this form as well. However, it was unclear for Frost the extent of the power he could attain with this form, and its power was never seen, as Frieza eliminated him shortly after.
100% Full Power shares almost exactly the same weakness as Super Saiyan Third Grade. Being that the increased energy causes the user's muscles to swell to a very large and bulky size. This bulk causes a significant loss in stamina and energy when used for extensive periods of time. And after a while, Goku pointed out that Frieza's energy was getting lower and lower with each hit he landed on Frieza. Despite the drawbacks, unlike Super Saiyan Third Grade and other Power Stressed forms, Frieza was able to retain his speed in this form.