Integration Services uses transformations to manipulate data during an ETL dataflow. Transformations can be used in Data Flow Tasks, between data source and destination components, or other transformations.

Sometimes sorted data flows can be a key requirement, especially before joining data with the Merge Join transformation. Two, previously sorted data flows can be merged into a single, still sorted one if they have the same sort metadata (the type and length/precision are the same, in the same order in both inputs).

This transformation supports exactly two inputs and a regular output. It does not support an error output.

The Merge is a partially blocking (asynchronous) transformation: the output is copied into new buffers, the input buffers won't be reused.

If sorting is not a requirement, consider using the Union All transformation only. If more than two inputs should have been merged, use a Union All first, then a Sort transformation instead of Merge.

For more detailed information on this transformation, please refer to this MSDN article.
For the complete list of SSIS transformations, check this Wiki article.