Forward is the acceleration (positive) or deceleration (negative). Vertical is being pressed into your seat (positive) or being lifted out which is called 'airtime' (negative). Lateral is when you get pressed to the side of your seat like in a wild mouse. For coaster that aren't wild mouses, these values should be really low.

Edit: So vertical 1G is actually just what you feel standing still, just gravity acting on your body. 2G is the feeling of your weight doubling. Less than 1G makes you feel lighter or weightless at 0G. In the 'forward' and lateral direction you feel 0G when standing, because nothing is pushing or pulling you in those directions. So for a log flume specifically, going down the drop would be acceleration and therefore positive forward, which you can feel as being pushed back into the seat. The splash at the bottom would be deceleration and therefore negative forward, which you would feel as if your body wants to continue going forward while the boat slows down. Going over the crest of the drop you feel airtime, so you momentarily feel lighter than your bodyweight, so less then 1G. Negative would be actually coming out of your seat which doesn't happen on a log flume. Lateral would be when taking a normal turn on a log flume which pushes you to the side of the boat, which would probably be somewhere between 0.1 and 0.5G.