In other words, give the characters in your story traits that people can identify with. As writers, we often dream about being other people. We want to be smart and beautiful and able to lift a train with our bare hands. We want to be special.
Too Much Perfection
In that quest, sometimes we write characters that are too perfect. They have no flaws. While I enjoy reading about a flawless girl who kicks ass, if there's nothing wrong with her, two problems exist: 1. I can't see myself in her. She's too close to perfect for me to imagine myself in her shoes; and 2. There's no room for character development. Good stories have a character who changes during the course of the plot.
Room To Develop
A character can be powerful or seem to have everything, but as long as they are lacking something--even if it's just one thing--there's room for that character to develop. Take Superman. He flies, is super-strong, bulletproof, handsome, has ridiculous abilities, and even has a job. A near-perfect person. He has it all, right? But he leads a lonely life. Never mind the kryptonite weakness, he has very a human problem anyone can relate to.
Redeeming Qualities
Now let's look at the opposite end of the spectrum: Bad guys with no redeeming qualities whatsoever. They're ugly, mean, and inhumanly cruel. I, as a reader, don't enjoy these kinds of bad guys because I hate them so much. They're so bad they don't seem believable, and it would be appalling if such a person actually existed. Give them a trait to humanize them. Maybe offer a piece of their past to explain why they're so mean and cruel. Or maybe they're mean and cruel to people, but they're kind to puppies. By humanizing your bad guys, it makes it seem like such a person could exist, and that makes the story more compelling.
Emotions
Even if you ignore all the above, one quick way to make characters relatable is to share their emotions with the reader. Humans are emotional creatures, and can identify easily with emotions in other people. If your character can express sadness, elation, or even boredom, we can relate, because we've felt all those things too. In any situation, make sure your character has facial expressions or other physical cues so the reader has an idea of what he or she is feeling about it. (Shoutout to AndresFelipeCurreaCa for inspiring this chapter update.)
Readers want relatable characters. Believable characters. If you give your characters at least one problem or one human trait that people can relate to--even if it's just emotions--then your story becomes more believable as well, and you're one step closer to finding an audience.
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Is this helpful at all? I'd appreciate a vote!