General liability (GL) is the foundational business coverage. It responds to three types of third-party claims: bodily injury, property damage, and personal/advertising injury.
Bodily injury: a customer slips in your store. Property damage: your contractor drops a tool through a client's window. Personal/advertising injury: a competitor sues you for defamation in a marketing email.
GL does NOT cover: your employees (workers' comp), your own property (commercial property), your professional mistakes (E&O), your vehicles (commercial auto), or cyber events (cyber liability).
Standard limits are $1M per occurrence / $2M aggregate. That's usually the minimum written into commercial leases and vendor contracts.
GL is what a certificate of insurance (COI) most often certifies. When a customer asks you to add them as an 'additional insured,' they're asking to be included under your GL policy for claims arising from your work for them.