Lambda cold starts occur when AWS Lambda has to initialize a new instance of a function before it can execute the code. This initial run of the function, which includes loading the runtime, your code, and the dependencies, is referred to as a “cold start.” The time taken for this initialization process is the “cold start latency.” In contrast, if an instance of your function is already running and is reused for subsequent invocations, it is considered a “warm start”.

Source: CloudTweaks