What does it mean "OutOfMemory"

Hello

I am doing load test on a server. I am increasing the load gradually.

I have done the Load test for the users 50, 100, 150, 200, 300. all are succeeded.

But when i try to do for 350 users, it gave an error "Out Of Memory" in the middle of execution.

Could any one  please tell me what does it mean "Out Of Memory"



I am using TestComplete 8



Please answer the question ASAP



Thanks

Ravi