form drag

I cannot understand the bold section. Please help me.

Form drag is created by any structure which protrudes into the relative wind. The amount of drag created is related to both the size and shape of the structure. For example, a square strut creates substantially more drag than a smooth or rounded strut. Streamlining reduces form drag.

a square strut (= strut with a square cross-section) creates a lot of drag (air resistance)

a smooth or rounded strut (= strut with smooth or rounded cross-section) creates much less drag

a “strut” is a long thin piece of metal, wood, plastic etc. that supports or strengthens a structure