What is Temporary impact, permanent impact, square-root impact law, percentage of volume?

The temporary impact of a trade is the part of its price concession that decays after the trade; the permanent impact is the part that remains, the information the market reads in it. An order’s percentage of volume is its size $Q$ over the market’s daily volume $V$. The square-root impact law states that an order’s average impact, as a fraction of price, is about $\eta\,\sigma\sqrt{Q/V}$ for daily volatility $\sigma$ and a constant $\eta$ of order one.