The disintegration rate of an element is the percentage of an element that has been altered. Generally, it reacts with its environment (oxidation, reduction ...) or it degrades spontaneously because it is not stable (radioactive isotope).
When half of an element has been altered, the disintegration rate is 50%.
The decay rate is often used to measure the half-life time of an element.
If we take 1 mole of Uranium 235 (i.e. 6,022.10^23 atoms), it spontaneously emits alpha radiation. After 703.8 million years, only 0.5 mole of Uranium 235 will remain (the rest will be converted into Thorium 231).