"Ó" (Ógift/Ógiftur) means "not married"
"E" (Ekkja/Ekkill) means "widow/widower"
"tómthúsmaður" basically means a fisherman, living by the seaside, who does not do any farm work.
A bit of info about the people in question:
Guðbrandur never married - he first had two children with Kristín, then five children with Pálína Margrét Jónsdóttir, and finally one child with Ólöf Einarsdóttir.
Kristín first had a child with Helgi Jónsson, then married Sigmundur Guðmundsson - they had two children (one of which was Jóhann Sigmundsson, yes), but he died in 1865, leaving her a widow, then she had one child with Guðni with Guðmundsson, then two with Guðbrandur (as mentioned earlier), and finally one child with Jón Jónsson.
Guðbrandur Samúel Guðbrandsson never married, but had one daughter, Guðrún.