What does it mean when a student confuses 'f' and 'v' in spelling?

Dr. Louisa Moats, author of the LETRS professional development program for teachers, talks about why students confuse 'f' and 'v', for example, in spelling and why going back to phonemic awareness activities may be necessary. She explains that these two sounds look alike and feel alike when you say them, but the difference lies is what your vocal cords are doing.