Others include AtScale[0], dbt's MetricFlow[1], Google's Looker[2] (also a BI tool but powered by a semantic layer), and Propel[3].
[0] https://atscale.com
[1] https://www.getdbt.com/product/semantic-layer
[2] https://cloud.google.com/blog/products/data-analytics/introd...
[3] https://www.propeldata.com
They're kind of an updated version of OLAP cubes if you're familiar with those.
Typically semantic layers sit on top of a data warehouse, let you define metrics using code or a UI, and provide APIs or SQL connectors so that you can query them.