This is a huge question, and I am afraid I don't have the time to do it justice at the moment. Charlemagne is probably the single most important political figure in the early medieval period and certainly one of the most important in all of western medieval history. Perhaps someone else will take up the gauntlet and really do him justice.
At its most basic, Charlemagne, or Charles the Great (from Carolus Magnus in Latin) oversaw a massive expansion in Frankish power and territory that would shape the political and territorial face of Europe for centuries. He was responsible, both directly and indirectly, for major intellectual and religious reforms and development in the eighth and early ninth century that shaped the progression of western intellectual thought and Christian thought. He was also, incidentally, the first figure in the west to bear the title of "Emperor" since the end of the Roman Empire in 476.
What I can give you is some places to start research if you want to go further and get specific.
Rosamond McKitterick's Charlemagne is a great place to start for both understanding the man and his importance in European history.
Pierre Riché's The Carolingians is a bit more text-booky and covers the entire period. It is quite accessible, though somewhat dated.
Paul Dutton's Carolingian Civilization contains a ton of primary documents in translation that can be a fun read, including a large section of Charles the Great.
Costambers, Innes, and MacLean's The Carolingian World is dense but very recent, thorough and good.
You can also find primary sources such as Einhard's Life of Charlemagne online open source or in a million different translations if you want to read a text by one of Charlemagne's contemporaries that is dedicated to how great he was.
Another fun read is Notker's Deeds of Charles the Great written for Charlemagne's great-grandson. Its an interesting look at how quickly Charlemagne became a legendary figure and gives you some idea of why he was considered important in his own time and beyond.