What is the Plug icon I see when trying to charge my macbook air?

If battery charging is paused or on hold on your Mac

The algorithm, coded into Big Sur, is designed to pause the actually charging process when the charger is attached and on power. It is supposed to lessen the number of Full Battery Cycles that would be incurred if the charging was permitted to go all the way to 100% charge each time the charger was used. This way, in theory, the battery should last longer before degrading in capacity and eventually requiring a replacement battery

It could be related to the battery health management. See this link https://support.apple.com/guide/mac-help/if-your-battery-status-is-not-charging-mh20876/mac

Regardless of the charger or cord used, you will see this icon periodically. It's not a problem; It's Big Sur's way of making sure your battery is optimized for performance and longevity.