What is linear distortion?
If you what a detail, and unintelligible definition you can refer to the wiki.
From the digital photography school: Linear Perspective refers to the way lines are drawn and captured to show an objects perceived size within space. In short, how we translate real world depth into a flat picture. Linear Perspective is made up of two basic concepts, the horizon line and vanishing point. The horizon line represents the viewing angle of the observer. Vanishing points are the point (on a horizon line) where parallel lines meet (converge). For example, the point where the two sides of the straight road meet (as shown below).
Or in other words, lines that are actually parallel do not appear to be parallel.
How do you correct distortion?
One way to minimize the effect is to keep the camera centered on the subject, and the shooting straight on (not angling up or down).
You can also adjust the image afterwards in programs such as Lightroom or Photoshop.
Thanks for reading, keep on shooting, and, as always I look forward to your continued support.