[jampekka]

That there's a probability of 4.20e-05 that the observed difference of that large would happen by chance due to observation noise if there would be no real difference (given your assumptions about the data generating process holds).

[tstenner]

The normality assumption is a large stretch, especially since there an absolute lower limit near the observation and a somewhat discrete distribution, so a t-test isn't appropriate. But then again it looks significant, so there's no real need for a test

[jampekka]

Something like Mann-Whitney U would be safer. But it's indeed quite obvious from the samples that the difference is robust.