What is Adverse-selection component?

The adverse-selection component of the effective spread is the part that compensates liquidity providers for trading with counterparties who know more: for a trade at $t$, the expected value of $\varepsilon_t(P^\ast_t-m_t)$, where $P^\ast_t$ is the efficient price, the expectation of the asset’s value given all information, including what the aggressor knows and the quotes do not yet show.