In the context that I was reading, it was related to discovering something major or finding out some incredible pieces of information. I've been struggling to find the right English phrase to use it for.
It is an exclamation sentence with a dirty word.
我 - the subject
了个 - have no actual meaning, just express exclamation.
大 - an adverb, expressing a strong feeling
槽 - a dirty word, the correct form is "肏(f**k)". The Chinese people like to use 我肏 to express surprise.
Please avoid using exclamations like this because they include impolite parts. But, the Chinese people like to use it very much in their daily life. It may be the most common exclamation sentence in spoken Chinese in daily life.
Also, I agree with the translation from @LiuYan 刘研.
I think 我了/勒个大槽/操/草/艹 is derived from 我勒个操/卧了个槽, which is again derived from 我操. They both mean 'Fuck', 'Shit' or 'Holy shit', but in different length of words. One can say 我勒个操 conveys a stronger emotion than 我操, and 我勒个大操 even stronger, but the differences are complex and closely related to the context. I believe a detailed analysis of the differences between 我勒个大草, 我了个草, and 我操 can be as long as a paper - this is really an interesting pragmatics topic.