Typically, a semantic layer is where you expose your data to downstream applications. Depending on your architecture you may apply business rules, denormalizations, transformations, calculations, etc. in the semantic layer. Reporting tools will typically connect to the semantic layer and ETL extracts for downstream systems often pull from the semantic layer.
SomeTeradata customers will have a single semantic layer for the warehouse while others may have multiple semantic layers either by subject area, accessing application, etc. They will all utilize the same set of data from the warehouse but may apply slightly different rules/transformations as needed.