An HDMI Splitter is a product that allows the connection of a single HDMI source to be produced into multiple displays. For example if you have a 2-Way Splitter it means the HDMI source will be turned into 2 outlets. 'Splitting' can be a confusing term because it would indicate that it splits the power of the source into 2, or more, but really there is absolutely no signal loss. It actually amplifies the signal before output so instead of 'splitting' the signal it really 'multiplies' the signal. There are both 2-Way and 4-Way Splitters on offer.
For information on the difference between Splitters and Switches, see our blog post "Splitters & Switches - What's The Difference?"