Scrolling around the selecting text is a basic operation, and there are a lot of commands for doing some variation of those things.
In Source Insight, a selection is a range of zero or more characters highlighted in a source file window. Selected text is highlighted and displayed in the "Selection" style; usually an inverse color. You can change the look of selected text by editing the Selection style in Options > Style Properties.
Each source file window has its own single selection. Switching between windows leaves the selections in each window intact. The selection in the current window and current file is referred to as the current selection.
Selections can be made with the keyboard and the mouse. Commands used for selecting text are listed below. To select text with the mouse, simply point at the text and drag across it. Double-clicking the left mouse button selects a whole word.
Scrolling the window will only change the selection if the "Keep cursor in window when paging up and down" option is turned on in the Options > Preferences: General dialog box. Otherwise, only cursor movement keys, pointing with the mouse, or editing your file will change the selection.