[ThereminVox]

Originally Posted by totoum

Not completely sure but the "ff" reminds me of a music symbol.

Yup. Musical notation. Two "f"s in that style mean "fortissimo" or "very loud" relative to the piece being played.

Originally Posted by MartianMage

Spoiler for animated blushing sayaka:

So here, for example, Sayaka is saying "Ahh, not so fortissimo, Kyouko-chan, what if someone hears us?"