Dots are used to represent broken rhythms.
The simplest way to represent the dot is by using:
> meaning “the previous note is dotted, the next is halved”
< meaning “the previous note is halved, the next is dotted”

Here’s an example:

X: 1
A>B C<D abcd

ABC is a flexible format and there are many alternative ways to represent the same musical sequence; for more information see The ABC standard.