Translation:What is this?
My understanding (and my Chinese isn't particularly great) is that 这个 is a pronoun where as 这 alone is a demonstrative. It can be difficult to understand because English doesn't differentiate (if you speak Spanish, it would be like 这=esto and 这个=ésto).
The more granular explanation is that 个 is one of a class of words known (i believe) as "counters" or "counting words" (they're EVERYWHERE in East and Southeast Asian Languages). Basically, when you are counting things there is a "classifier" that you have to add (think, like "200 head of cattle" in English). The word 这 is a demonstrative meaning "this"... When you add a classifier (here: 这个) you are no longer just saying "this," you are saying "this one (of this type of thing)".
I hope that didn't just make matters worse... And PLEASE correct me if I'm off fellow learners!!!