What is it
Cloud Storage is our (and GCP's) go-to solution for schema-less storage. You create "buckets" with a classic file/folder structure.
When to use it
When you want to store any kind of file in the cloud. It can be images to be used on a web page, data dumps that we want to store for future analysis, or temporary files for ingesting data into BigQuery. Buckets can be used as a source to BigQuery, making it a good "data lake" solution. I.e. we can dump unstructured data that we may want to analyse later on.
How to learn it
Read the Quickstart.