A bin is a group of values which are counted together, when using an insight query to produce statistical reports.
When you query a representation which returns continuous values - that is to say, which can have any value between a minimum and a maximum - then the possible values are grouped into ranges. For example, the Employment Range representation of the Age category groups values into 0-18, 19-25, 26-35, and so on. Each of these ranges counts as a bin.
When you query a representation which returns discrete values - that is to say, which can only take a value from a pre-defined list - then each of the possible values counts as a bin. For example, the Region representation of the Address category returns strings like London or Scotland, and each of those strings counts as a bin.
The aggregated statistics returned by the query will report on the number of values in each bin.