Ah, yes, the dreaded leftover part after reassembly.
I replaced the hot end on my Taz 5 (upgraded 0.35mm to 0.5mm) and have this suspicious little square bracket leftover. Thanks for advice on what this thing reinforces.
That extra square block is the retention plate for the heater cartridge and thermistor. It is confusing, as in your photo I can see yours is successfully installed. I suspect it came off of your old hot end, and you can see where it would go under the install retention plate step here: https://ohai-kit.alephobjects.com/project/cf3b0638-1e1e-4bff-a00e-c3308d914c68/
NOTE: NEVER run your hot end without that plate installed, it is an important safety feature.
Ah! It did come off my old part. Thank you.
No problem! Enjoy the new hotend!
Brent, after reassembly my LCD reads, “Err: MAXTEMP”
I’ve ensured that everything is plugged in correctly. I’m assuming the thermistor isn’t working as expected.
Err, never mind. I had motor plug and hot end plugs reversed. 
we have all been there, glad you got it sorted!