It is the closest thing a chain has to a user count, and it is not a user count. One person can control hundreds of addresses, an exchange can serve millions of customers through a handful, and bots generate activity indistinguishable from people.
Read it as a trend rather than a level, and against the chain's own history rather than across chains, since cheap-gas networks inflate easily and expensive ones suppress genuine use. Sustained divergence between price and active addresses is the pattern analysts watch, in both directions.
Manipulation is cheap where fees are low. Addresses are free to create, so a project wanting to look busy can generate activity for the cost of gas. Pair it with value transferred, fees paid and onchain-volume before drawing conclusions.
Related: onchain-volume, on-chain, throughput-tps, crypto-address