What is a service mesh? This is a great article that explains service meshes.
20191126/https://servicemesh.io/
Now, for a discussion in my own words. Basically, a service mesh allows you to throw more stuff onto the technology heap without rearchitecting the whole system. Definitely not a good idea if you are designing a small systems project, but like many other similar technologies, this facet makes it wildly popular in the large complicated software systems sector of modern cloud computing, enterprise back-end, etc.