A VU represents a user of a service or application. A VU definition is usually made up of a number of actions that represent a typical usage scenario.
When testing a web app a VU would usually map to a person using that application. For a GraphQL or RESTful API a VU would represent an API client. For a Kafka cluster, a VU would represent a producer, and so on.
An SLI is measurement of some aspect of the performance (or service level) of a service, usually expressed as one or more numeric metrics.
For example, availability is a common SLI, which for a simple HTTP-based service may expressed as:
number_of_successful_requests / number_of_total_requests.
SLO is a goal for an acceptable performance level of a service, measured with one or more SLIs.
For example, an availability SLO for a simple HTTP-based service may be 98% availability as measured by the ratio of successful requests to total requests in a sliding 10m window.
An SLA is an explicit contract between a service and its users. An SLA is based on one or more SLOs and usually specifies the consequences of missing those SLOs.