The Fed usually seeks a 2% inflation as a comfort zone.
Granted, it's only one data point but with unemployment currently low and some signs that economic growth is accelerating, the suggestion exists that overheating/inflation is a risk for the U.S. economy in a way that hasn't existed for about ten years.
What is inflation?
Inflation is when the buying power of a currency declines over time. If inflation is 2% that translates to a basket of groceries which cost $100 today costing $102 a year from now.
But since prices rise and fall and we each buy different articles how is that tracked? Government statisticians and economists create indexes to reflect a full range of products and services that are consumed weighted by how much the average household spends on each item.