\/
The \/
command produces an italic correction. This is a
small space defined by the font designer for a given character,
to avoid the character colliding with whatever follows. The italic
f character typically has a large italic correction value.
If the following character is a period or comma, it's not necessary to insert an italic correction, since those punctuation symbols have a very small height. However, with semicolons or colons, as well as normal letters, it can help. Compare f: f; (in the TeX output, the `f's are nicely separated) with f: f;.
Despite the name, roman characters can also have an italic correction. Compare pdfTeX (in the TeX output, there is a small space after the `f') with pdfTeX.