They are talking about the electrolyte - the material between the battery anode and cathode that allows the transport of lithium ions.

A normal lithium ion battery generally uses gels-like materials. Lithium hexafluorophosphate dissolved in organics is common, and is a viscous liquid.

A solid state electrolyte is basically a solid ceramic membrane that allows the ions to travel through it. Many potential benefits around stability, sustainability, and durability. Challenges around electrical contact, particularly as the battery cycles because the active materials do change volume slightly as the lithium moves between the sides of the cell.