Here is another table of fundamntal forces as used in particle physics.
Fundamental means the basic framework, and the basic framework is quantum mechanics, from which macroscopic forces emerge which can be demonstrated mathematically. These forces exist at the particle level, and their strength relative to each other enter in the Feynman diagram description of the complicated integrals that have to be computed in order to describe data at the particle level.
The column called "strength" gives the coupling constants that have to enter in a multiplicative manner whenever a vertex in a feynman diagram involves the corresponding force. The coupling constants are measured from data. Example here for weak and here for electromagnetic. Mesurements are fitted by specific calculations, and thus the valuce of the coupling constants are defined.
The relative strength is gotten from data that need two different couplings, so the Feynman diagram calculations are fitted to the data, and the relative strength is determined.
In a hand waving way, the diagrams with the weak constant gives small crossections with respect to the ones with electromagnetic vertices, that is why the were called weak. The calculations quantify this.
Here is a link for measuring the strong force coupling.
Gravitation is still not definitively quantized, but the coupling constant is used in effective quantizations. Here is a link on how it is defined.