It would be interesting to know the context upon which you came across the term 'metric'. That will help answer your question.
But in layman's terms a metric is often referred to as a 'numeric value' used to measure something like sales, profit, calls handled etc... There are more strict definitions of a metric but mine is a very simple answer.
Usually metrics are reported at a detail level and are usually in a fact table however this is not always the case and certainly isn't a rule.
From the best of my knowledge, any numerical field which is used to measure is a metric. Generally numerical's are metrics. It is a best performance practice to have them in a fact table. In some properly created data model they give a # symbol before the field names for identifying it a metric used for calculation.
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