Impedance, measured in ohms (Ω), describes how hard a speaker or headphone is to drive. It isn't a single fixed number — it varies with frequency — but speakers are rated with a nominal figure, usually 4, 6 or 8 ohms. Lower impedance means the amplifier has to supply more current, so a 4-ohm speaker with dips toward 2 ohms demands a beefy, current-capable amp.
For headphones the story is similar: high-impedance (250–600Ω) models often need a dedicated headphone amp to reach satisfying volume and control, while low-impedance IEMs can be driven by a phone but may then reveal the phone's output noise.
The practical rule: match the amplifier's capability to the load. An underpowered amp into a difficult speaker is the most common cause of "thin," strained sound at volume.