In Latex, the command \underline is for adding the under line effect in your text. For example,
This is an under example.
However, if you use this for multiple lines, the line wraps will disappear, the multiple lines come to be one unique line.
To solve this, you can use the ulem package.
And for highlighting your text by under line, use the \uline command.
This will provide you the under line effect for multiple lines.
A drawback of this method is that it changes the default emphasis behaviors. All the other text highlighted by command \emph will be underlined too. This might not be what we want. For example, in revtex, emphasis is shown by the italic font.
To make the ulem package not affect the default behavior of emphasis, use the following command to import the package:
Now you can use the \uline command without worrying about the default of the emphasis behaviors.