What does the term "irresistible grace" mean?

“This is what we mean when we use terms like sovereign grace or irresistible grace. We mean that the Holy Spirit is God’s Spirit, and therefore he is omnipotent and sovereign. And therefore, he is irresistible and infallibly effective in his regenerating work. Which doesn’t mean that we don’t resist him. We do. The Bible is plain about that (Acts 7:51). What the sovereignty of grace and the sovereignty of the Spirit mean is that when God chooses, he can overcome the rebellion and resistance of our wills. He can make Christ look so compelling that our resistance is broken and we freely come to him and receive him and believe him.” ~John Piper

Source: symphonyofscripture.com