A hairline crack, in simple terms, is a crack that hasn’t opened yet. It’s there, it’s compromising your basement’s structural integrity, BUT it’s not a threat yet. On the other end of the crack, there might be water building up, but it’s not leaking into your basement. Again — just yet. Hydrostatic pressure, the expanding and shrinking soil (especially if the area where your house is located has a lot of clay in the soil), and many other factors will eventually lead to the crack expanding.
Every hairline crack, if not sealed and repaired on time, will inevitably become a full-scale dangerous crack. It’s just a question of time.