Something effervescent has bubbles or froth, like a sparkling cider or a bubble bath. If you have a happy, light, cheerful personality — if you are "bubbly" — you too are effervescent.
Coming from the Latin effervēscere, the original meaning was more boiling than bubbly, thanks to the ferv part, which means "hot." The word literally refers to bubbles giving off gas, and any drinks you enjoy that are nose-ticklingly effervescent make bubbles that pop and release into the air. Remember, someone with an effervescent personality is just bubbly, not full of gas!