Volado comes from the bottom of the plant: little flavour, but it burns readily, and it is there for combustion as much as for taste. Seco comes from the middle and carries most of the aroma at moderate strength. Ligero comes from the top, thick and oily and the strongest of the three. A blend is a proportion of the three, and changing that proportion is most of what blending is.
Why ligero sits in the centre
Because it burns slowest. Placed at the core with faster-burning leaves around it, the cigar burns evenly; placed at the edge it would lag and the coal would run crooked. The physical arrangement inside the bunch is a burn decision as much as a flavour one.
Medio tiempo
On some plants a further pair of leaves grows above the ligero. They are small, rare, and not present on every plant in a field, which is why cigars built around them are made in limited numbers.
The names are about position, not variety
All three can come from the same plant of the same seed in the same field. This is the point most often missed: they are primings, not varieties.