That is a bit of marketing speak that simply means the insulation on the wire will not permit current to flow under some unspecified differential voltage. The insulation is simply black silicone rubber and it is rated for a maximum working voltage of 750 volts.
As an antenna, the silicone rubber insulation will have no practical effect on the efficiency or effectiveness of the antenna. If you are cutting the antenna to some specific length assuming a bare wire, you can take 3% or so off that length to account for the dielectric effect of the insulation.
Unlike many wires insulated with common PVC (e.g. THHN and XHHW), the black silicone rubber should not suffer from UV degradation. So unlike THHN, for example, you won't go out in your garden only to find pieces of insulation lying about. As @MikeWaters points out, the silicone rubber is also generally resistant to ozone which may help to keep the insulation intact if your antenna happens to be exposed to high ozone levels.