What does SHITA mean in japanese?

Shita した can mean the past tense of suru to do, or it can just mean below.

it is the past tense version of "suru" which means do, "suru" is the present form of do and "shita" is the past tense of to do meaning did. した, however, can vary in meanings depending on context and can also mean under as well下.
total mindfuck sometimes, hope this helps.