Our reptilian brain is the most primitive and oldest part of our brain. Among other jobs, it is mainly concerned with regulating physiological homeostasis.
It regulates heart rate, breathing, and digestion.
It regulates the level of overall excitation of the nervous system.
It is concerned with basic survival and security needs and is responsible for fight, flight and freeze responses. Actually, there are variants to the basic flight or fight reactions that are just as basic.
It is the job of our reptilian brain to help us feel physically oriented in space-time of our immediate situation.
When we experience something that is either actually dangerous, or reminds us of some past experience of danger, our autonomic brain immediately gets the whole nervous system geared up for some variation of the basic fight, flight, freeze safety procedures.
Unless we have a robust relationship with our own reptilian reactivity, we cannot reset our patterns of reactivity. Our reptilian brain cannot be fooled. It will go on protecting us and managing ground level patterns of self-maintenence until a reliable partner – ours, or a trusted other higher cortical brain – shows up and demonstrates that it will be reliably present to and honor the concerns of the reptilian brain.