"הסינתיסייזר הוא כלִי נגינה מוזר."
Translation:The synthesizer is a strange instrument.
"By itself כלי is something like "implement," yes?"
Yes, and coupled with other words, it can "morph" into other words. כלי נגינה = musical instrument(s - depending on how you say it, this spelling can be singular or plural). כלי עבודה = tool(s). כלי בית is one you'll often see on storefronts - that refers to housewares. In a spiritual context, כלי refers to our abilities and attributes. I'm sure there are many more! כלי is an awesome, useful word.