Wondering about distances in the Universe.

Sometimes, scientists use terms that the general public has no idea what it means.  Now, I have always been interested in sciences, especially astronomy, but I am confused by some of the terms.  Just like last October, the Carnegie Observatories reported that their most recent measurement of the value of the Hubble Constant (the expansion rate of the Universe) is 74.3 +/- 2.1 kilometers per second per megaparsec.  OK???  I was going along with them just fine until I got to the "megaparsec" part.  You all probably know what it is but I have no idea.  I was wondering if it had anything to do with "light years", which is a logical definition of distance.  One light year is the distance that light (which travels through space at a constant speed) travels in one year.  Although that, too, is hard to comprehend (like all distances in the universe) it is something that is in more commen use and we all seem to understand what is meant by it.  OK, so tell me, what is a megaparsec?

Well, let me try to explain.  This is something that I didn't know so I am learning this right along with you.  As the earth circles around the sun once a year, it makes the closest stars to us apear to shift back and forth against the rest of the stary background.  Scientists can use this shift to determine distances to these stars.  That process is called a trigonomic parallax.  Stars shift less than a second of an arc (you know, a degrees, minutes, seconds) which is about about the width of a quarter seen from three miles away.  To see this shift in a star, it would have to be 3.6 light years away.  This is the distance of one parsec.  Got that??  I think we should stick to the old reliable light year, what do you think?  Now, please don't worry too much about megaparsecs but have yourself a mega-good day teday, you hear?