We need more information. Exactly what were the symptoms and what did the mechanic do. If he’s commenting on a compression test, that may be an inability to hold pressure or an inability to achieve sufficient compression, each with causes of its own. If he commented based on his “reading” of a spark plug (like one plug crudded up or damaged from too much heat) then the possible causes would be different. If you have one cylinder misfiring, that’s a whole different range of possibilities.
While my first suspicion is that he was referring to compression, it’s hard to tell from the info provided.