This bundles the Bioconductor object schemas into an R package, so that it can be easily referenced by other packages without requiring internet connectivity. If you're looking to do something useful with these schemas, check out alabaster.base instead.
This package is available from Bioconductor, so if you must install it manually, you can do so via the standard Bioconductor installation process:
# install.packages("BiocManager")
BiocManager::install("alabaster.schemas")