Question
What is a service mesh?
Answer
In the context of Kubernetes, it is something that "provides infrastructure services for traffic management, observability, and security" for container connectivity (as explained on page 230 of Kubernetes Patterns by Bilgin Ibryam and Roland Huß (O'Reilly). Copyright 2019 Bilgin Ibryam and Roland Huß, 978-1-492-05028-5). A service mesh will often use a sidecar. "…most meshes use the sidecar model…" taken from Platform9.com.
"A service mesh refers to the way that software code from cloud hosted applications is woven together at different levels of the webserver in integrated layers." (This quote was taken from VMware.com.)
"The mesh provides critical capabilities including service discovery, load balancing, encryption, observability, traceability, authentication and authorization, and support for the circuit breaker pattern." taken from https://www.nginx.com/blog/what-is-a-service-mesh/
Service meshes have applicability outside the realm of microservices (according to page 1 of Service Mesh for Dummies).
"In short, service mesh tools like Istio can reduce the operational burden of managing microservices-based applications, and in particular traffic between services, which could otherwise involve significant and often unsustainable manual work." taken from https://enterprisersproject.com/article/2019/6/service-mesh-plain-english
Spoiler alert: See #58 in this link to Answers to Kubernetes Quiz for examples.