> And before I start I just want to be sure, a LED driver is just another word for a constant current source right?
Most off-the-shelf LED drivers are some form of constant current driver.
An LED can technically be driven by anything: constant voltage, constant current, constant power & even more complex systems:
CV driving: possible, but fiddly. Has to be very accurate and under-drive the LED, otherwise risks blowing it up as the LED warms.
CC driving: works, stable, popular.
CV + resistor driving: works, stable, popular. Technically not CC, but close enough & requires less parts if you already have a known CV supply.
CP & other more complex driving schemes: not very popular, simpler methods typically work fine.
> if I want to power multiple of these from the same PSU do I just get a 300ma constant current PSU that has a higher voltage?
Yes
> From my little understanding of how these work they simply work the same as constant voltage, but instead of using voltage as feed back, it uses a shunt for current, and it will increase/decrease voltage as needed so that the draw is whatever it's rated at correct?
Yes.
Rather than thinking in terms of "reducing" and "increasing" voltage I suggest you think in terms of "more V and I" and "less V and I". For most loads (including LEDs): increasing either with increase the other.
Some CV supplies can be turned into CC supplies by modifying where they measure their feedback voltage from.
> And lastly, lot of drivers tend to just use PWM instead of a smooth DC output, can I safely stick a capacitor at the end to smooth that out or will that mess up the current measurement somehow and cause it to deliver wrong current?
No you can't just stick on a cap. It will charge quickly to full in the ON period and basically make lowering the PWM do nothing (except increase current draw over short periods, which increases stresses and can cause damage). You will instead want to put a proper filter on the PWM output (ie L + C), which effectively turns it into a switchmode power supply, but this requires some calculations & considerations to get right.
EDIT: PWMing LEDs can be anything from harmless to problematic. Technically driving them at a constant (variable) power instead of a PWM waveform is more efficient, but in most cases people don't care enough to pay for the extra parts. This also shifts the colour temps of white LEDs slightly.
Of course there's also the problem of when the PWM freq is so low that people and/or cameras notice it
