An Attack Pattern is an Intelligence Object with many Relationships which you will deal with a lot in your work.
According to STIX Version 2.1, Attack Patterns are a type of tactics, techniques, and procedures (TTP) that describe ways that adversaries attempt to compromise targets. Attack Patterns are used to help categorize attacks, generalize specific attacks to the patterns that they follow, and provide detailed information about how attacks are performed.
An example of an Attack Pattern is "spear phishing" which is a common type of attack where an attacker sends a carefully crafted e-mail message to a party with the intent of getting them to click a link or open an attachment to deliver malware.
Attack Patterns can also be more specific; spear phishing as practiced by a particular Threat Actor (e.g., they might generally say that the target won a contest) can also be an Attack Pattern. (Reference: STIX Version 2.1)