Stock prices work like most other free market items, the greater the supply, the lower the price. On the flip side, the greater the demand, the higher the price. Corporate stock buy-backs have historically provided a tailwind for higher stock prices as companies retire shares (thereby increasing demand while reducing supply) but have been decreasing for the past 3 years.
It looks like 2018 may reverse that decline. If the above repurchase forecast turns out to be true, it should provide a bullish backdrop for equity prices this year. While it may or may not be enough to push prices higher, it will surely provide a floor likely keeping a lid on the size of any decline.