 Solution

An $L$-embedding $\pi :A\to B$ is an injective map preserving the interpretations of all constants and functions and preserving and reflecting every relation. Explicitly $\pi (c^A)=c^B$, $\pi (f^A(\bar{a}))=f^B(\pi \bar{a})$, and $A\models R(\bar{a})$ if and only if $B\models R(\pi \bar{a})$. Injectivity handles logical equality. Equivalently it is an isomorphism onto an $L$-substructure of $B$, or a map preserving the truth of all quantifier-free formulas. Preserving relations in only one direction would instead give a weaker homomorphism notion, not an embedding.