Acorrding to Genesis 6:3

“My Spirit will not remain in humans forever, for they are mortal; their days will be a hundred and twenty years.”

This verse is not understandable for me:

First question - What does it mean "spirit"? Is it the Holy Spirit or is it "the breath of life" in Genesis 2:7

Then the Lord God formed a man from the dust of the ground and breathed into his nostrils the breath of life, and the man became a living being.

Second question - What was the reason(s) for not remaining "the spirit" in human? Increasing in population? and why God decide to do so much later than He creadted first mankind?