Paht_Hed,

They mean a 1-1-1 ratio between your G,M and B bottles relative to each other. The strength/concentration depends on how much you actually add in "parts".

Read the recipe since the "part sizes" are usually different. Some common ones are:
1 part = 8ml/gal
1 part = 1tsp/gal (5ml/gal)
1 part = 10ml/gal

Soooo...assuming 1 part = 8ml, your "half-strength" of 1-1-1 would be:
GROW = 1 part * 8ml * 0.5 = 4ml Grow per gal
MICRO = 1 part * 8ml * 0.5 = 4ml Micro per gal
BLOOM = 1 part * 8ml * 0.5 = 4ml Bloom per gal

Say you wanted "half strength" 0-1-2:
GROW = 0 parts * 8ml * .5 = 0ml Grow per gal
MICRO = 1 part * 8ml * .5 = 4ml Micro per gal
BLOOM = 2 part * 8ml * .5 = 8ml Bloom per gal