Can you help explain this definition of a sigma bond

I know what a sigma bond is; however I crossed paths with this definition of a sigma bond during my studies:

A head on overlap of orbitals which have a circular cross section normal to the internuclear axis. If the two orbitals do now have matching symmetries, no bond can be formed.

I understand the head on overlap part, but what does it mean to have a circular cross section normal to the internuclear axis?
What about the matching symmetries part? How does a p orbital have matching symmetries with an s orbital?

Thanks in advance.

Normal is another word that means perpendicular. The internuclear axis is an imaginary straight line between the two nuclei. Since the two s orbitals are spherical, when they intersect, you should be able to draw a straight line through them that is perpendicular to the internuclear axis. If you look at this textbook http://www.ochem4free.info/YQcr3l7Abrssp… , the illustration on page 32 will really help.

I'm confused about "If the two orbitals do now have matching symmetries" part. Did you mean "If the two orbitals do NOT have matching symmetries"? If that is what you meant, all that's saying is that an s orbital cannot bond with an p orbital. Orbitals can only bond with orbitals that are their own type.