The longer a coin stays still, the less likely it is to move soon. On-chain analysis uses this by sorting the supply by age. The common dividing line is 155 days: coins younger than that belong to short-term holders, older coins to long-term holders.
What the two groups do
Long-term holders tend to buy when prices are low and sell into strength, so their share of supply usually rises through bear markets and falls as bull markets peak. Short-term holders are the opposite: more recent buyers, more sensitive to price, and quick to sell at a loss when markets drop.
Coin days destroyed
When a coin moves, the days it sat still are "destroyed". One coin idle for 1,000 days destroys 1,000 coin days, the same as 1,000 coins idle for a day. A spike means old coins have moved, which often gets attention. But it could also be an exchange reorganising its own cold storage, so check whether labelled entities explain it.
The 155-day line is a convention
Different windows give different pictures. What matters is reading the same definition consistently over time, and asking whether a change comes from coins moving or simply from coins getting older and crossing the line.