A SIFZ file is a .SIF project compressed with gzip and created by Synfig Studio, an open source application used to create professional 2D animations. It contains an animation, which consists of a canvas filled with shapes, brush or pencil strokes, text, and position, radius, tangent, angle, vertex, and width handles.
SIFZ files are one of three file formats used by Synfig Studio but are also the most common format used. Since they compress their contents they are smaller in size than uncompressed SIF files that often become bloated. Synfig Studio also uses .SFG files for embedding images, audio, etc. into the project which is compressed with Zip compression.