"-inf" stands for "negative infinity" meaning no sound. Think of it as the same level as muting the track. For 16- and 24-bit integer format files it can literally mean -96 dBFS and -144 dBFS, respectively, since that is the lowest recordable levels for those formats. Different applications may define "-inf" differently. I think Sonar faders will say "-inf" at around -75 dBFS.