There are three types of concurrency in Lambda

(1) Unreserved concurrency

(2) Reserved concurrency

(3) Provisioned concurrency

Explanation

• Unreserved concurrency: The amount of concurrency that is not allocated to any specific set of functions. The minimum is 100 unreserved concurrency.
• Reserved concurrency: Guarantees the maximum number of concurrent instances for the function. 
• Provisioned concurrency: Initializes a requested number of runtime environments so that they are prepared to respond immediately to your function's invocations. This option is used when you need high performance and low latency.