The creation and customization of database applications using Microsoft Access
What is the purpose of this? As thedbguy said, having a Boolean (Yes/No) column in the table, and a check box control in the form bound to the column, would enable you to 'select' multiple rows. You could then use an SQL statement to manipulate the data in the 'selected' rows, if that's what you are attempting, or you could use conditional formatting to identify the rows visually.
More information about the desired aim would help us advise you further.