It is somewhat unfortunate that Peter Abelard is known for his juicy and scandalous tryst with Heloise, because he was one of the greatest thinkers of the twelfth century. His dates are: 1079-1142. In terms of his significance, there are many, but I will name a few of them to get you started.
First, many see him as the greatest scholastic theologian of his time. In other words, he was at the height of rationalism for this time. And from an intellectual point of view, he brought Aristotle front and center within Christian theology. This was quite a shift, because Plato for a long time held the ascendancy. Abelard gave rise to people like Thomas Aquinas.
Second, his work on the atonement was a huge step forward. For example, before Abelard, there were two theories of the atonement. First, there was the ransom theory. This theory basically stated that Satan overstepped his boundary when he killed Christ. In light of this, Jesus was able to ransom his people. Second, there was the satisfaction theory, which stated that Jesus restored the honor of God through his death and resurrection. Instead, Abelard argued that Jesus died on the cross to demonstrate limitless love. This teaching was banned and Abelard barely escaped with his life. However, later on the church would view him more charitably.