In the course of an implementation or during productive activities, one of the hardest things to manage is Master Data, because it is difficult to administrate and the level of importance is high.
The administration of this master data is needed in case of an implementation from a non sap system or when you need to create large amounts of data. In both cases creating the records in a single screen can be a task that could take months.
An LSMW is a program that records the inputs that a user writes in a specific screen, and from a spreadsheet or a CSV, imports those same fields into the transaction.
This process is repeated as many times, as records are.
For more information and to learn how to create a LSMW refer to UDEMY and search for the course Learn in SAP how to upload Master Data (LSMW)