When a buffer has errors, the ErrorList plugin displays a global overview of all errors in a vertical bar to the left of the text area's vertical scroll bar. Each error is shown as a thin horizontal line. The color depends on the error type; genuine errors have a red underline, warnings have a yellow one. Clicking on an error will scroll the text area to that location. Hovering the mouse over an error will show a tooltip with the error message.
The error overview can be disabled in the Error List pane of the Plugin Options dialog box. The colors of the highlights can also be changed in that same option pane.