What is meant by transfer of data from legacy code to SAP?

It all depends on how frequently you are accessing SAP data. There are
several ways you can transfer data Legacy data (non sap system) to SAP. You have
list of choices

1) There is option in SAP called BAPI by using which other SAP and NON-SAP
systems can interact.

2) Another option is IDOC/EDI concept

3) Exchange Infrastructure (XI)

4) BDC - Batch Data Communication

5) LSMW etc.