Privilege escalation
Gaining a higher level of access than you started with, such as moving from a standard user to an administrator.
Vertical escalation increases your rights on a system; horizontal escalation moves you sideways into another user's account at the same level. Attackers pursue both, because a standard user account rarely reaches the data worth stealing.
The usual routes are unpatched local vulnerabilities, misconfigured service permissions, credentials left in scripts or shares, and overly generous group membership that accumulated over years and was never reviewed.
Where this comes up
Privilege escalation sits inside our cybersecurity and threats work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.