A BigQuery slot is a unit of computational capacity required to execute SQL queries. BigQuery automatically calculates how many slots are required by each query, depending on query size and complexity.
See the quota policy for queries for the per-account slot quota.
Most users find the default slot capacity more than sufficient. Access to more slots does not guarantee faster per-query performance. However, a larger pool of slots might improve performance of very large or very complex queries, as well as performance of highly concurrent workloads. To check how many slots your account uses, see Monitoring BigQuery Using Stackdriver.
BigQuery automatically manages your slot quota based on customer history, usage, and spend. For customers with at least $40,000 in monthly analytics spend, BigQuery offers several ways to increase the number of allocated slots. Contact your sales representative for more information.