In the short run, it's quick absorption into the bloodstream jacks up your insulin level, which paradoxically makes your blood sugar crash, making you feel tired and hungry, and leading you to want another sugar-rush for the momentary burst of energy.
Over time, it causes high blood pressure, insulin resistance (leads to diabetes), excess fat storage, liver damage, increased vulnerability to inflammations, and suppresses the brain's reception of the enzyme that says "now you've had enough," thus encouraging constant eating in a second and more powerful way.
In other words, it's bad stuff. Avoid it as much as possible.
Get the energy you need in slower-paced, more healthful ways from foods that "package" their carbs with fiber: whole grains and fruits (not fruit juices, which have all the sugar of fruits with none of the fiber).
Complex carbs (from whole grains and starchy vegetables like potatoes) are turned to a kind of sugar (glucose) by the digestive process, but without the array of damaging effects caused by adding refined sugars (sucrose and fructose).
Here's a good short lesson in the biochemistry of sugar consumption, with a link (just below the video, in the description) to the much longer version that it summarizes:
If you prefer print to video, you can also find the same basic info here:
I think its converted into a short burst ofenergy.