It means the statistical confidence interval includes 0% efficacy. This implies the result of the trial was not statistically significant. This is a pretty good indicator that the intervention, in this case wearing a mask, doesn't provide any significant benefit.
I have no idea what this means "the error bars stretch to zero"
It means the statistical confidence interval includes 0% efficacy. This implies the result of the trial was not statistically significant. This is a pretty good indicator that the intervention, in this case wearing a mask, doesn't provide any significant benefit.