What do those numbers on the outside of rear shock springs mean?

Usually designated in the imperial system (well for the brands I deal with anyway) rear shocks have 2 numbers (e.g. 600 x 2.00). The first number (600) is the spring rate. The rate of a spring is the amount of force necessary in pounds to compress the spring one inch. In our example it takes 600 lb. force to compress the spring one inch. So it will take another 600 lb. (total 1200) to compress it the second inch.

The second figure (2.00) is the travel of the spring in inches and corresponds to the maximum travel of the shock.

Note: Fox confuse spring marking by quoting the maximum travel the spring will go to before it becomes coil-bound. eg 1.65 = 1.5 stroke, 2.35 or 2.38 = 2.0 or 2.25" stroke, 2.8 = 2.5 or 2.75" stroke & 3.25 = 3.00" stroke

Fork springs are also available with different spring rates but only have one number (e.g. 60). This is the spring rate; 60 lb. force to compress the spring 1 inch. Fork springs have much lower spring rates as they are connected directly to the wheel with no mechanical leverage involved. Rear shocks normally work with ratios of between 2 and 3.5 to 1. So on a bike with a 3:1 ratio, when the rear wheel moves 3 inches the rear shock moves 1 inch. If a 600 x 2.00 spring was fitted it would take 200 lb. of force at the rear wheel to move that inch. The front fork on the other hand, with its 60 lb/in spring rate would take 180 lb. to move the same 3 inches.