ESXi is a hypervisor developed by VMware that allows multiple virtual machines to run on a single physical server. It is a type of bare-metal hypervisor, meaning it installs directly on the hardware without needing a host operating system. This enables efficient resource management and isolation between virtual machines. Administrators use ESXi to create, manage, and monitor virtual environments, making it easier to deploy applications and services. It supports various operating systems and provides features like live migration, snapshots, and high availability, enhancing the overall performance and reliability of IT infrastructure.