Strength represents the physical amount of force a being can produce in proportion to it’s mass. The range for strength is from 0 to 7.
The force exerted is usually by the species main manipulating appendages. For example, a Human’s arms or a Takolee’s arms or legs would be manipulating appendages. A Human able to do a chin-up would be a good example of someone with at least average strength, since he would be lifting his own mass (assuming 1g).
It should be stressed that the strength rating is only in proportion to the beings mass. For example, a one tonne beast with a strength of weak (1) is still going to be able to exert more force than a 20 kg being with a strength of strong (6). In contests of strength, it is the force produced that really matters.
To find the strength of a being, roll an appropriate die for the range of strength show in the species descriptions. This number is used to determine the force the being can exert, but it also says something about the appearance of a being. A being with a strength of 6 is going to have very visible muscles, while a being with a strength of 1 isn’t.
Once the strength of a being is rolled, you must determine the force the character can exert. Multiply the force percentage by the species average mass, multiply by 10 and you have the force (in Newtons) the being is able to exert. In a 1G environment, skip the multiply by 10 step to find out how many kg of mass the character can carry.
Or, you can look up your species and strength on the Force Table.