It's a way of boasting or bragging or showing off. If you do something difficult you can say it. For example, if someone is trying to impress his friends, he might say:

"You know, I once killed a bear with only a knife."

His friends might express disbelief. "No way! You killed bear with just a knife? Weren't you scared? I know I would be!"

"Yeah, well, it was nothing I couldn't handle," he might say, smugly.

Another way to use it is to compliment someone. If your boss gave you three big stacks of paper work and you managed to finsh them, she might say: "good work today. I knew the extra work was nothing you couldn't handle."