Answer:
Underestimating is when you round something down and overestimating is when you round something up.
Step-by-step example:
estimate 0.57x6.8 .
Over-estimating: 0.6x7
Under-estimating: 0.5x6
A better way to understand this is that OVERestimating is going OVER, and UNDERestimating is going UNDER ( or really staying the same)
Have a great day