they are the limb thickness.

one word as additional information:
Those weights written on the limbs are the weight you would pull if you pulled 28" long.
So lets say you have a 66" long bow with limbs written "30#" on them (at 66" long) then you wouldnt pull 30 lbs at your drawlength. with 26" you are 2" shorter than the convention. So i would go for 32# @ 66" to get around 30 lbs drawn at 26" instead of 28".

the risers normally come in 3-4 different length's:
21"
23"
25"
27" (New)

the limbs come in 3-4 different length's:
(children)
Short
Middle
Long
Limbs differ in 2" each step except children...

If you combine a 25" riser with a long limb, you get a bow of 70" length.
If you combine a 25" riser with a middle limb, you get a bow of 68" length.
If you combine a 25" riser with a short limb, you get a bow of 66" length.

Same game goes with the risers of 27", 23" and 21".

in the example
Samick Polaris 62"/66" Limbs 62" 39-40/66" 37-38 +$53.99
it will be a 21" long riser with either long or short limbs.
With the 21" riser + long limbs you get a bow of 66" with a weight of 37-38 lbs @ 28" drawlength.
With the 21" riser + short limbs you get a bow of 62" with a weight of 39-40 lbs @ 28" drawlength.

If i had to choose (as we do for our youngest members in the club) i would go for a
Samick Polaris 21" (gives 62"/66" bow) with long limbs to get 31-32 lbs @ 28" draw. with your 26" that should be in the ballpark of 29-30 lbs drawn.

The weight can be adjusted a bit (in the case here yround 1 lbs) with the big hand-screw holding the limb to the riser.

62"/66" is the length of the bow limb 62 inches or 66 inches
the 37-38 and 35-36 is the pounds draw.
so the 62" 37-38/66" 35-36 means
the 62" limb bow is 13 to 40 lb draw and the 66" limb bow is 13 to 38 lb draw stringht.

so if you want the 30 lb draw weight you want 62" 29-30/66" 27-28 or the 62" 31-32/66" 29-30.

Joe