Thrust to weight ratio is simply the pull of the engine compared to the weight of the airplane...

The weight of the airplane is determined by the total mass of its components and fuel, as affected by the gravitational pull of the earth.

The thrust is determined by the power of the engine being converted to airflow by the propellor. Thrust is normally measures as "Static thrust" where the aircraft has no forward speed. When the aircraft is moving its possible for the thrust to decrease or increase... dependant of the enging and propellor. (Jet engines and ducted fan engines always give more thrust when they have the "ram air" from the forward motion of the aircraft.)

Thrust to weight gives a partial indication of expected speed and/or aerobatic ability of the aircraft. A low thrust to weight (1:4 or less) and the aircraft could still fly but would be hard pressed to do aerobatics. A high thrust to weight ( 4:5 or higher) is needed for extensive aerobatics.

A full scale Piper Cub can fly on a 35 hp engine (which is what they put in the original J-3) With a 85 hp engine, the (otherwise nearly identical...) PA-18 "Super Cub" is much faster and more aerobatic.