What Does 8 x 20-µs and 10 x 1000-µs Mean in Surge Testing Terminology
"8 x 20us" means a transient that rises in 8us and decays to a 50% value in 20us: -



There are many specs for transient testing and 10 - 1000us is another. They are usually referenced as tests done on electrical equipment to prove said equipment is capable of withstanding surges associated with lightning strikes or similar phenomenon.

Here is a good article from Wiki EIG (electrical installation guide)