Overconsumption means buying more than you need. It becomes a problem for sustainability when people buy much more than they need, because this means that lots of resources are being used to satisfy what one person wants. Resources are not infinite (there is not a never-ending supply of materials, such as wood or plastic), and it takes energy and human labour to make, ship, and sell products.
In short, what we choose to buy has an impact on the world. When we choose to buy a lot more than we need, we are having a larger impact on the world, and this impact is likely to be more negative than positive.
Things to try:
Consider borrowing items rather than buying them, if this is possible;
Shop second-hand to give another life to items, such as clothes, that already exist in the world;
Think carefully before buying something or asking for something as a gift – it is okay to want things, and owning things that feel special to us can help improve our quality of life, but we will appreciate things more if we make careful decisions about what we would like to own.