Time series files (tiser) are human-readable text files in two column format (the first column encodes time and the second column encodes the value). These files are for instance read by FileModulatedPoissonGroup and are typically interpreted with linear interpolation between data points. That means they are interpreted the same way how you would plot them by connecting all the dots by a line.
For example, the file:
0.0 1.0 1.0 1.0 2.0 20.0 10.0 5.0 10.0001 1.0 20.0 1.0