Monoubiquitination is the addition of one ubiquitin molecule to one substrate protein residue. Multi-monoubiquitination is the addition of one ubiquitin molecule to multiple substrate residues. The monoubiquitination of a protein can have different effects to the polyubiquitination of the same protein. The addition of a single ubiquitin molecule is thought to be required prior to the formation of polyubiquitin chains. Monoubiquitination affects cellular processes such as membrane trafficking, endocytosis and viral budding.

Reference
Wiki: Monoubiquitination