For the average player it doesn't actually mean anything. Minecraft Java Edition uses Java as the name implies. Java 8 means version 8 of Java, this is the default version used by the official launcher.
The forge + 2 thing is due to CurseForge displaying the version numbers in a really bad and annoying way. Forge itself is treated as a version number instead of as a mod loader or project type. So a mod for forge which supports 1.12.2 will be forge + 1. The Java version is treated similarly, so if they say java 8 (the default) then it is forge + 2. You may have to click on the file itself to see the real version.
This is a known issue with the launcher that affects some players. The easy way to fix it is for you to download the file using the link they give you and then place it in the folder on your Computer. In this case you would download https://libraries.minecraft.net/net/sf/jopt-simple/jopt-simple/4.6/jopt-simple-4.6.jar and then place it here C:\Users\evpel\Twitch\Minecraft\Install\libraries\net\sf\jopt-simple\jopt-simple\4.6\jopt-simple-4.6.jar