Indexer
Software that reads a blockchain block by block and reorganizes its contents so applications can query them quickly.
Nodes are built to validate, not to answer analytical questions, so asking for every transfer of one token by one address over a year is impractical against raw node storage. An indexer decodes transactions and contract events into structured tables and exposes them through an API; The Graph's subgraphs and the in-house pipelines behind exchanges and analytics sites are both examples. The consequence is that almost every chart a reader sees is the output of indexing logic, and a decoding bug, an unmapped contract, or a missed proxy upgrade appears as missing or wrong history. Chain reorganizations force parts of the index to be rebuilt, which is one ordinary reason on-chain figures are revised after publication.
In practice
Two analytics sites can report different totals for the same protocol because each decodes a different set of contracts and events into its tables.
The common misunderstanding
Disagreement between dashboards about a public blockchain usually reflects different indexing and decoding choices, not ambiguity in the chain itself.