A sole proprietor is an individual who owns an unincorporated business by themselves. For workers' compensation, sole proprietors (along with partners and some corporate officers) are often excluded from the employee count and are not automatically covered by a workers' comp policy, though they can usually elect to include themselves.
This matters for two reasons: it affects whether a business hits its state's employee threshold for mandatory coverage, and it determines whether the owner's own workplace injury is covered.
Example: a sole proprietor with no employees is generally not required to carry workers' comp, but can choose to buy coverage on themselves so their own on-the-job injury is protected.
Related: workers' compensation, exclusive remedy. See the workers' compensation insurance overview.