The thrust-to-weight ratio is a simple ratio of thrust to weight of a flying object such as a rocket. It can be either a maximal or peak, average, or minimum thrust-to-weight ratio. Most fliers use the average, but the minimum can be just as important when determining if a flight might be stable. The calculation is performed by dividing the thrust (Newtons) by the rocket’s weight (Newtons).
The calculator will calculate the larger number of the ratio or X. The X to 1 or X:1 ratio is what you are looking for to estimate stability. The thrust will change during the burn. For this reason, a 5:1 is a suggestion by many experienced rocket flier and builders. This is not arbitrary but is suggested for a reason. I have launched with a lower ratio but often the flight ends with skywriting (unstable flight).
Higher is better to a point. Too long of a rocket or too low of thrust can lead to weathercocking into the wind. Too low, you can never guess where it’ll go or how many pieces it will end up becoming. Again, 5:1 is a suggestion. It will allow you to survive both weather and engine