Kerning is where you adjust the spacing between characters in a piece of text so the letters are not too far apart or not too close together.
Below are some homorous examples of why we need to check letter spacing.
10 Flickering Lights.
Massage Therapist (A little less space between the E and the R I think).
The pen is, mightier than the finger.
The Wig and Pen is open for business.
Click Lovers, Computer Specialitst
For a Special Aunt.
Not sure what they're going for on this one, but still very funny.