Good, simple explanation of Service Level Objectives (SLOs)

SLOs are objectives that your business aspires to meet and intends to take action to defend; just remember, your SLOs are not your SLAs (service level agreements)! You should pick SLOs that represent the most critical aspects of the user experience. If you meet an SLO, your users and your business should be happy. Conversely, if the system does not meet the SLO, that implies there are users who are being made unhappy!

Source: Building good SLOs – CRE life lessons