A non zero click remote code execution would be for example the attacker sends the victim a message, with a link or attachment, that if the victim interacts with it, the attacked gets to run code they wrote in the victims device.
A zero click remote code execution, would be for example where the attacker send a message, and their phone just processing the message on it's own is enough for the attacker to execute code on the victims device.
A non zero click vulnerability can be mitigated by being cautious. A zero click vulnerability cannot.
An exploit that achieves remote code execution just by a browser performing an HTTP request (for example a malicious ad) would be considered a zero-click exploit.
A zero click remote code execution, would be for example where the attacker send a message, and their phone just processing the message on it's own is enough for the attacker to execute code on the victims device.
A non zero click vulnerability can be mitigated by being cautious. A zero click vulnerability cannot.