Apple - Why is an album in Apple Music marked with an "E" and grayed out?

The E symbol means "Explicit".

You can enable or disable explicit content within Settings on your device. You'll find the "ALLOW MUSIC & PODCASTS RATED" option under General -> Restrictions -> Music & Podcasts.

You may need to enter a PIN code to access the Restrictions settings.

You can disable the restrictions entirely by selecting the "Disable Restrictions" option at the top of the Restrictions window.