This is the 4846th most frequent Portuguese word.
fio
Thread; wire; string; filament.
In this sentence, "fio" refers to a physical wire or cable used for conducting electricity.
Eu comprei um rolo de fio para usar na instalação elétrica.
I bought a spool of wire to use for the electrical installation.
Here, "fio" metaphorically represents the continuity or sequence of the conversation.
O fio da conversa começou a se perder.
The thread of the conversation started to get lost.
In this case, "fio" means a thin strand or filament, specifically a strand of hair.