all very confusing
conventionally, to send the audio signal to an outboard amplifier, you use RCA audio cables. this setup works via the so-called 'low level' audio signal; that is, 'low voltage' (typically <4V). it is low level because the signal does not pass through any amplifier inside the headunit.
'high level' audio signals are high voltage, and can be up around 8~10V at full volume. the signal has passed through the inbuilt amplifier inside the headunit. basically, this is the signal emitted via the 'speakerwire' outputs, to run speakers directly off the headunit. the inbuilt amplifiers are somewhat crude, and therefore the signal is more distorted and prone to noise, typically audible as background 'hiss'.
ideally, you use RCA outputs from the headunit to connect an amp. but the standard headunit does not have these. therefore, you have to rely on the high level speakerwire outputs. not all amplifiers are compatible with this, though all are compatible with low level RCA connections.
to utilise high level connections, you either need an amplifier with an inbuilt high level input (different to RCA inputs). entry level amps usually have these.
alternatively, use an external converter - aka LOC (line output converter or line level converter or hi-lo converter). these accept the high level connection from the headunit, then drop the voltage to low level, and have RCA outputs for connection to an amplifier. the signal is only as good as the weakest link, and having already passed through the inbuilt amplifier IC in the headunit, it still isn't as good as a direct RCA output that an aftermarket headunit will offer.
LOCs - external or inbuilt into amps - can vary in quality too.
decent LOCs are by Navone, Alpine, and Stinger. there are also quite sophisticated processors which combine an LOC, EQ, and other processing; for example the Rockford Fosgate 3sixty series.
for an inexpensive, basic stereo, you might choose an amp with an inbuilt LOC, and connect it straight to the factory speakerwires (which requires finding the factory wires, extending them, and connecting them to the amp). unless you have a really good understanding of car audio, the car, and autoelectrics, it's best done by a professional car audio installer.