The definition
A skills inventory is a record of the skills currently held across an organization, at what level, and by whom. It differs from a skills matrix mainly in scope and intent: a matrix is usually one team, read as a grid, and used for gap analysis; an inventory is organization-wide and used to answer whether a capability exists anywhere before assuming it has to be hired.
It is the input to several decisions that are otherwise made on instinct. Whether to build or buy a capability. Whether a project can be staffed internally. Which parts of the organization are one resignation away from losing something important.
Why freshness is the whole problem
Every skills inventory is accurate on the day it is compiled. What happens next decides whether it was worth compiling. People learn things, change teams, and stop practising what they used to be good at, and none of that reaches a spreadsheet unless somebody is paid to keep it current.
An inventory refreshed once a year is wrong for eleven months of it, and — this is the damaging part — continues to be quoted with confidence during those eleven months. A stale inventory is worse than none, because none prompts someone to go and ask.
Making it maintain itself
The only durable answer is to make the inventory a by-product of something people already do. If levels are set by assessments taken as part of normal development, the inventory updates as a consequence of the work rather than through a separate data-collection exercise.
That also fixes the accuracy problem underneath. An inventory built from self-declared skills records what people believe about themselves; one built from verified levels records what they have demonstrated. The gap between those two is consistently larger than anyone expects before they measure it.