Bottom line
The Uniformed Services Employment and Reemployment Rights Act (USERRA) is the federal law that protects your civilian job when you leave it for military service. If you meet the eligibility rules, your employer has to take you back, treat you as if you had never left for seniority, and not discriminate against you for serving. There are hard limits and deadlines: your cumulative military service generally cannot exceed five years, you have to give your employer advance notice before you leave, and you have a set window to apply to come back when your service ends.
What it means for you
This is your safety net every time you go on orders, including drill. Tell your employer before you go (verbal or written is fine). Keep your service under the cumulative five-year cap. Come back or reapply on time based on how long you were gone (the deadlines below). If you do those things and you were released under honorable conditions, your job, your seniority, and your health insurance are protected by federal law. If an employer fights it, you have free help available through Employer Support of the Guard and Reserve (ESGR) and several enforcement paths.