What does "zero-meaned vector" mean
"Zero-meaned" means the vector has been transformed so that its mean is 0.

Typically, you would do this by subtracting the mean of each column from that column. (This is for dimensional as well as algorithmic reasons; you don't want to subtract a person's weight from their height.)

It sounds like here they're actually talking about the row mean--that is, $(-0.6946377, 12.680544,  0.50395286)$ would be transformed to $(-4.857924, 8.5172577, -3.65933344, 4.1632863, 7.40047)$, where the first three are the original features minus the row mean, the fourth is the row mean, and the fifth is the standard deviation of the original features.

This would make sense if the three have the same units (if they're all accelerations at the same scale, this works), and so you want a separate measure of how much it's being accelerated at all and how much it's being accelerated in a particular direction.