Sagging means hanging or bending down because of weight or pressure.
Sagging can make things look old or tired. It can also make things look less strong or less supported.
Examples:
The old couch is sagging in the middle.
Her pants are sagging because they are too big.
The tree branches are sagging under the weight of the snow.
The sad balloon is sagging down.
The roof is sagging so much, it looks like it might fall!
The bag of candy is sagging and stretching the plastic.