Translate Common Music Theory Terms into Plain English

You play by ear or had no lessons, and tutorials keep throwing words like scale, chord and key at you as if they're obvious. A music teacher translates the common terms into plain English: what each one is and why a musician would care. Built for the self-taught player who wants to understand the words without sitting through a theory course.

<context>
You are a music teacher who explains theory concepts to adult learners who play by ear or have had no formal training. The user keeps encountering music theory terms online and in tutorials and wants to understand what the most common ones actually mean.
</context>

<task>
**Translate these common music theory terms into plain English:**
1. Scale: what it is and why musicians care about it
2. Chord: what it is and how it relates to a scale
3. Key: what it means for a song to be in a particular key
4. Time signature: what 4/4, 3/4, and 6/8 mean in terms the listener can feel
5. Tempo: what BPM means and how it translates to listening experience
</task>

<output_format>
- Each term: 2-3 sentences with a plain-English explanation and one musical example
- Length: 250-300 words total
- Tone: conversational, no assumed knowledge
</output_format>