Can you quickly summarize it in easy words?
Anti Evil Maid basically lets you know if there were any unauthorized changes to your system, such as if there was an evil maid attack.
And can you recommend installing it? I wonder why it isn’t installed by default if it increases security.
Anti Evil Maid is not just a straight-forward security upgrade. There are some security considerations/tradeoffs to take into account before setting it up. Anti Evil Maid requires connecting a USB device to dom0 which is generally not recommended because it is a potential security risk.
From the Anti Evil Maid documentation https://www.qubes-os.org/doc/anti-evil-maid/:
This presents us with a classic security trade-off: each Qubes user must make a choice between protecting dom0 from a potentially malicious USB drive, on the one hand, and protecting the system from Evil Maid attacks, on the other hand.