Answer:
We are assuming the balloon has been rubbed by a cloth, giving it extra negative charges.
After the balloon has been rubbed, it gains a negative charge because it gained some negative charges from the the cloth. This means there are more negative charges than positive ones to neutralize the effect, so the balloon gets a negative charge.
Due to the law of charges that states "Like charges repel each other; unlike charges attract," when the negatively charged balloon is brought near a wall, the wall's negative charges are repelled and pushed away from the balloon. Meanwhile, the positive charges in the wall are attracted to the balloon's negative charges. The strength of this attractive force is enough to keep the relatively light balloon attracted to the wall, which may sometimes keep it suspended in its place.