Series and signals can represent the same. However, a signal will be defined as a number or observations at a fixed sample rate, whereas a series would not have to. One example could explain it. Imaging two consecutive R peaks (which mean two consecutive heartbeats) then an RR value can be calculated as the time difference between them. The RR value corresponds to the first heartbeat, so extending the idea one RR time series has values unevenly sampled due to the time beat occurrence.
Series can be generated to store time events, such as RR, or to store amplitude, such as series of T wave amplitudes. In addition, pre-processing series is also available in BioSigBrowser.