A metric is a quantitative measurement. It's the stuff that you're counting. You use metrics to quantify measurements within your data. For example, if you want to measure the traffic to a website, one metric you could use is page views. Another could be unique visitors.
Metrics work together with dimensions. A dimension is an attribute of your data. It's how you group your data. Back to the example of measuring traffic on a website, you might want to know which pages get the most traffic. In that case, page title or URL could be your dimension.
In Interana, metrics are called Measures, and dimensions are called Groups.