A ruled surface is a surface in whose parametric equations one of the two parameters appears only at the first degree and with a single arc (in this case, parameter v).
The isoparametric curves relative to parameter u are therefore straight lines, which are called the generators of the ruled surface.
In other words, ruled surfaces are obtained by joining the u points of equal value on two base curves with a suitable set of straight lines.
To create a ruled surface, use the Lofted Surface command.