What they're trying to say is that the sheet resistivity of the material is 30 mΩ/⬜ for a sheet thickness of 25 μm. However, that wouldn't be expressed the way they wrote it--it should be 30·25 mΩ·μm/⬜, or a bulk resistivity of 750 nΩ·m, or 75·10⁻⁸ Ω·m*. This seems reasonable for silver ink, as it's a little worse than most metals (between about 1·10⁻⁸ and 20·10⁻⁸ Ω·m), but substantially better than semiconductors (graphite, for instance, is very roughly 20000·10⁻⁸ Ω·m).
You can divide this number by the sheet thickness to get the sheet resistivity.
*The reason I use 10⁻⁸ Ω·m, rather than μΩ·m or nΩ·m, as a unit here is because this is what metal resistivities are usually quoted in in the literature. It works out to small numbers greater than 1 for most metals, and makes it easy to compare without having to move decimal places around. I think nΩ·m would have been a better unit to use overall, being a standard SI prefix, but the decision to use 10⁻⁸ Ω·m was likely made well over a century ago, before SI prefixes went smaller than μ.
⁻⁸ No, this is an exponent, not a footnote. What are you doing down here?