Yes, CUDA_LAUNCH_BLOCKING=1 is a debug env variable used to block kernel launches and to report the proper stacktrace once an assert is triggered. You should not use it in production, but only during debugging.
Kernel launches will be blocked, i.e. they won’t be executed asynchronously anymore as already explained. From CUDA’s programming guide:
Programmers can globally disable asynchronicity of kernel launches for all CUDA applications running on a system by setting the CUDA_LAUNCH_BLOCKING environment variable to 1. This feature is provided for debugging purposes only and should not be used as a way to make production software run reliably.