Having confidence means you believe in yourself; you trust your own judgment and resourcefulness. In his many books on self-esteem, Dr. Nathaniel Branden defines self-esteem as the sum of self-confidence and self-respect. For him, self-confidence is knowing that you have the wherewithal to function reasonably well in the world.
You feel competent to make choices, competent to satisfy your needs, to chart the course for your life. Having confidence in specific situations, such as in gaining influence with someone, would flow from a general self-confidence about your ability to meet life's challenges.
A person who exhibits confidence appreciates a sincere compliment and doesn't brush it off. A confident person is comfortable giving, and receiving, compliments. He's also able to handle criticism if it comes his way because he basically likes himself and knows that a single negative incident won't change that.
Confidence in yourself gets built up over time. You can fake confidence, and you may need to at first, but real self-confidence comes from a history of small victories and accomplishments that add up to a sense that you can handle yourself well in most every situation. I suggest you take an inventory of the major accomplishments you've achieved over the past few years. Then remind yourself of the minor ones too. What about the computer course you completed? Have you built anything that is still standing? What about those kids you're raising? That's (!) an accomplishment. Don't be modest. Tell the truth about how hard you worked, what sacrifices you've made. If you can't think of any, then begin by congratulating yourself for living as long as you have. Sheer survival is an accomplishment these days! Seriously, it pays to take the time to know your strengths and appreciate them. What's unique about you? What skills do you bring to an organization or project that you can count on?
Confidence is a fundamental trait for flexibility. It's hard to be flexible when you're fearful, or easily intimidated. Confidence is indispensable if you want to engage someone's attention.