Okay, this is a pretty interesting and advanced topic, but I'll explain it as quickly as possible.
Loudness is one tough thing to achieve without destoying your mix with limiters and distortion. But why do you want to achieve these crazy loud mixes?
Many of the modern streaming services such as Spotify and YouTube have their own loudness normalization systems in place. These systems basically lowers the volume of your track if it is too compressed and loud.
Why did they do this?
Well they're basically saying "f*ck the loudness war. Let's end it once and for all" (and maybe they don't want to scare their listeners and viewers with crazy loud sounds).
This is a really great thing for many producers though. When people listen to music, they're always going to prefer the louder songs. But imagine a classical orchestral piece compressed to sh*t. That wouldn't be very nice now, would it?
But okay, back to mastering and loudness. Here are some terms you might have heard of.
RMS stands for Root Mean Square, which are some fancy words. But why do you care about RMS? There is a new standard. LUFS.
LUFS stands for Loudness Unit Full Scale. This is just another type of average loudness metering. There is an recommended LUFS standard (mainly for radio broadcasts) that is set at -23. This is way too quiet for streaming services and mastering for club systems.
Let's look at an example. Spotify has a target LUFS at -14. Master over this value and their system will try to lower the volume of your track to make it match the rest of their library. YouTube has set its limit at -13 LUFS (at least in 2017).
Should you target these LUFS values? Well sure, why not. If you're making music that aren't targeted for clubs and festivals, it certainly gives your tracks lots of dynamic range and will sound super smooth.
What it won't give your tracks though is that competing loudness compared to other artists in DJ sets and on Soundcloud (or other streaming services that doesn't use loudness normalization systems).
In this Reddit thread you can read about someone talking about artists mastering their tracks to -3 LUFS. If you have a really great mix in a fitting genre (dubstep, big room etc.) this is totally possible. But if you're producing let's say Lofi Hip hop? Don't go for -3 LUFS.
Well okay. Here's what you should do.
Reference tracks should ALWAYS be used when you're mixing. Just pick a few tracks that matches your genre and style, and mix so that your track matches their. These can also be used for mastering. Check their LUFS levels with a LUFS metering tool and try to get your track just as loud as their.
Just make sure that you're getting the track in its highest quality. Don't use a YouTube to mp3 tool! Use beatport or iTunes instead.
I hope this helped you a bit on the way. Now go master your stuff and have fun. Bye.