I’ve seen two definitions for this term that I think are equally valid:
1—the language of casual conversation and informal writing
2—usage particular to a particular region, culture, or group
Common forms of colloquialisms include slang, jargon, abbreviated usage (emoticons, lol, leet, etc.), and even regional variations (sack vs. bag, coke vs. soda).
Many people view non-local colloquialisms as non-standard or even uneducated. The truth is, however, that everyone uses colloquialisms, and they have no bearing on education level, intelligence, social class, whatever. It is true that class distinctions do have some bearing on your colloquialisms, but your colloquialisms do not guarantee class.
What does it mean for me?
The most important thing for you is to just understand that colloquialisms exist and that everyone has them. You should also understand that many people view non-local colloquialisms negatively. Basically, it means that what you say has a bearing on how others perceive you. Whether that matters to you or not is your own choice.
In your dealings with others, you should probably avoid the most negative colloquialisms like “ain’t.” On the other hand, if you ever encounter a colloquialism, instead of looking down on or labeling the person negatively, take the time to figure out why they use it and how it works. And if that doesn’t help you overcome the negative tendency, it might just be time to pull out that wonderful old saying: “If you can’t say nothing nice, don’t say nothing at all.” (Which, by the way, includes the colloquialism of using double negatives!)