What does this symbol mean?
Il Decamerone, 1527 (available for free on google books)
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That would be the corresponding of present-day “&”, a glyph originated from a cursive ligature as a single character of an “e” and a “t”, to form Latin conjunction “et”, that is, “and”. And indeed it was and is still used in Italian, English and other languages to mean “and”. It's usually called “ampersand” in English and e commerciale in Italian.
Image from Wikipedia
If we look at the first page of the “Prohemio” (modern Italian, “Proemio”), we see
which shows some peculiarities. The most prominent is the usage of “&” instead of et. But we also see usages of “h” which modern Italian has dropped (humana, haver, havuto, hebbe) because it doesn't correspond to a sound (except in the verbal forms ho, hai and hanno).
The conjunction et was already pronounced e when in front of a consonant Altissimu, onnipotente, bon Signore, tue so’ le laude, la gloria e l’honore et onne benedictione (Cantico di S. Francesco).
During the middle ages, a wealth of ligatures and abbreviations were used: “&” for “et” was very common (also as abbreviation for “et” at the end of words).
It may look strange that in the first line of the text we find “et” and not “&” like in the other places. My guess is that using “&” would have posed typographic problems, but it shows that the usage was interchangeable.
The form you see is the italic version. This is how it's rendered in EB Garamond, that's based on almost contemporary fonts to the one used in the book.
Other curiosities are
The book was printed in London, and maybe the printer did not know Italian very well and confused “si” with “fi” (second line in the title)