What is Security Level In ASA Firewall?

Security level define to the Firewall Interface, Firewall Security Level can be 0-100. Where 100 is the highest security level on ASA firewall and most trusted Zone, By default its define to the Inside Interface. 0 is the lowest security level on the ASA Firewall, Its a define to the untrusted zone, such as Outside interface.

By default traffic allow from Higher Security Level to lower security level and traffic from lower security level to higher security level by default denied.