A time series is a sequence of data, modeled or measured, representing regular intervals over a period of time (several years in the case of a multi-year time series). Solargis multi-year time series is most typically used for following purposes:
A sample multi-year time series data file can be downloaded from here: http://solargis.info/doc/_files/sample_data/SolarGIS-climData/SolarGIS-hourly-time-series-PSA-ES.zip
A Typical Meteorological Year (TMY) dataset is derived from a multi-year time series.
TMY data is primarily used for energy simulation purposes, as popular simulation software such as PVsyst, SAM, etc. typically work with 8760 hourly values representing a typical year. The main reason for the popularity of TMY dataset for solar energy simulation is compatibility of such data with popular energy simulation software and speed of simulation.
However, as preparation of TMY data results in loss of information, it is recommended to use a multi-year time series for energy simulation purposes when possible.
A sample TMY data file can be downloaded from here: http://solargis.com/assets/sample/tmy/Solargis-SAMPLE-TMY-P50.csv