Here is the (maybe abridged) definition from the book - Artificial Intelligence, a modern approach

An inference algorithm 𝑖 is sound or truth-preserving if it derives only entailed sentences

It is complete if it can derive any sentence that is entailed

However, this does not make much sense to me. I'd like someone to kindly elaborate on this.

In section 7.3 of the book Artificial Intelligence: A Modern Approach (3rd edition), it's written

An inference algorithm that derives only entailed sentences is called sound or truth-preserving.

The property of completeness is also desirable: an inference algorithm is complete if it can derive any sentence that is entailed.

However, this does not make much sense to me. I'd like someone to kindly elaborate on this.