Back in the days when software was usually delivered on CD, there would be two types of CD - write-once and re-writable. Write-once CDs were silver in colour and are the ones you normally see music on (or at least you used to). They're cheap, and they're mass-produced. The problem is that once they're written once they can't be changed.
Re-writeable CDs, on the other hand, were much more expensive; and were gold in colour rather than silver.
So it was common, when software was ready to be released, to first write it to a gold CD. This CD would be a byte-for-byte copy of the intended release version of the software, down to the installer and everything. Basically there is no difference between the gold CD version and the final release version.
This gold CD version then gets a last round of testing to ensure that there is nothing wrong with it and that it is acceptable to release as a finished product. If it's fine, the same CD image gets written to the silver release CDs. If there is a problem, the gold CD can be erased, a fixed version of the software (or its installer) produced, and that new version is written to the gold CD for a repeat of the final testing.
From this (very common) methodology came the phrase "going gold". When software goes gold, it means that it is being declared as the finished product, with no more changes intended to be made. It will still get some final testing done, and if any showstopper problems are found they'll be fixed, but assuming nothing new is found it is the version that will be released - with any existing problems being classed as "known issues" that may or may not be fixed in post-release patches.
Basically, the process is (usually):
- Development - the software is being written here, and is not yet usable.
- Alpha Release - the software isn't finished, so it will still be having features added or changed, but it is working "enough" to be given to people to start testing. Usually alpha testing is internal to the company, but in the case of Early Access it is publicly available. However, the software is not expected to be feature-complete or stable, and will have many known issues. We are currently nearing the end of this phase with 7 Days to Die.
- Beta Release - the software is now feature complete, but is likely to still include bugs and other issues. It's close enough to being finished that what is now being tested is (barring unforeseen circumstances) similar to what will be released once it has been bug-fixed. Beta testing is usually done in-house like Alpha testing, although often there will be a "public Beta" where customers will be given access to the software during this phase. Obviously, with Early Access titles, customers have already had the software since the Alpha phase. 7 Days to Die is approaching this phase, and there is talk that A17 might be the last alpha and the next major release might be the first beta.
- Gold Release - as described above, this is when beta testing has finished and the developers are confident that they have fixed everything they need to fix. Whereas Alpha is expected to be unfinished and Beta is expected to be bugged, once the software "goes Gold" it is expected to be exactly what the finished product will be - and this is the last chance for testers to report bugs that might be big enough to delay release until they are fixed. There is talk that 7 Days to Die will probably reach this phase in 2019.
- Final Release - This is the same as the gold release, and is the finished product. At least, it's the same as the last gold release (if something major was found in the first gold release then it might have needed fixing and a second gold release made).
