The long-run average is the simple mean of every published reading. For a series with decades of history it captures several full economic cycles.
Treat it as context, not a target. Structural change can shift where a series settles: interest rates averaged far less in the 2010s than in the 1990s, for example.
Why it matters
Many economic variables are mean-reverting over long periods: very high unemployment eventually falls, very low inflation eventually rises. The average gives a reference point for how far from normal things are.
General information, not advice.