Explain the term 'reptilian brain' to me
 

I keep hearing it, and have no clue what it means.

It is the part of your brain that is at the base of your skull emerging from your spinal column. It is responsible for basic functions like breathing, heart rate and the "fight or flight" mechanism.

Read more about it here.

Also referred to as "thinking with the stem."

You have:
Reptilian Brain:
Instinct, fixed actions, emotion.
Formed from: amygdala, thalamus, hypothalamus
Amygdala is mainly emotions, thalamus integrates your senses, hypothalamus deals with stress responses and hormones.

Palaeomammalian Brain:
Behavioural flexibility and learning/memory (basic). Found in lots of mammals. Combines with reptilian brain to form limbic system. Contains hippocampus (memory, esp spatial)

Neomammalian Brain:
Language and higher thought. Made of the right and left cortex

Hope that helps!

I couldn't have said it better than clishe...
but Ill flap my trap anyway.

The idea of it being a "reptilian" brain is because it is a structure that we share with reptiles. (actually its most of what the reptiles have) It's mostly concerned with the basics of keeping you running from day to day. That is dealing with low level input (i.e. pain), low level output (i.e. swallowing) and simple motivations (i.e. hunger) Stuff you just can't do without if you want to keep your meat sack running about.