SSIS error outputs are a secondary path through which the data flow can send rows that do not conform to data type, length, or transformation standards defined by the ETL developer. That’s a lengthy way to say that it’s where you can send your junk data. In the SSIS designer, clicking on a source or transformation will often show not one but two possible outputs: the primary output (the “good” data, indicated by the blue line) and the error output (identified by the red line). As shown on the flat file source below, when selecting a source or transformation, those that have an available error output will appear with both output connectors ready for selection.
Tim elaborates quite a bit on what you can do with this output.