I have few doubt about MIL & SIL Test

Hi, I developed one model, and I have tested model using Signal Builder. Generally in this method, we will pass some inputs(test case) and get output. So this method call as MI test. Here I have double,some persons asked me, What is Model In Loop (MIL)? and What is mean by "In Loop", why we called as In Loop?. If input is depend on their output?

Can you please give some idea about it???