What does proportional mean in physics?

Question

so for example "force of gravity must also be proportional to the mass of Earth"

what does that mean?

Answer 1

Proportional means that something is equivalent to two properties in some fashion. For example, consider the equation V = IR, where V is the voltage, I is the current, and R is the resistance. Where one value is unknown, but the other two values are known, the relationship is used to determine the unknown value. With the voltage being proportional to the current, the constant is the resistance.

Answer 2

Proportional in physics means that there exists a relationship between two physical properties, the ratios of which is a constant of proportionality.  

For instance, force is said to be proportional to acceleration.  In this case, Newton told us that the constant of proportionality is the mass of the object upon which the force acts.  As such, in physics (and math) it is usually said that two things are proportional when this constant is not known.  When it is known we can then say that the two objects or properties are equal, ie; F = ma.