Explain this

one of the Fundamentals of Algorithmic Problem Solving is "Coding algorithm"

(Write in a programming language for a real machine, Standard tricks: (Programming Pearls by Bentley), Compute loop invariant (which does not change value in the loop) outside loop, Replace expensive operation by cheap ones)

explain what inside braces, I don't know the meaning of Pearls by Bentley and Compute loop invariant? as well what is expensive operation?