Statistics is concerned with the analysis and interpretation of data. Statistical methods are applied frequently throughout biology, from planning experiments to validating results and drawing inferences.
Use this tag if the use of statistics is a major part of your question, e.g. if you want to know whether a particular statistical test can be applied to your experimental results. Do not use the tag if the use of statistics is incidental. For example, if your question was "Why did the average height of an adult American male increase over period XXXX-YYYY?" you would probably be concerned with matters of nutrition, genetics, health and so on rather than how the data were collected and the averages calculated, so the statistics tag would not be appropriate.
Stack Exchange has an active site, Cross Validated, dedicated to statistics. If your question is primarily about statistics rather than bioloy, or requires dedicated statistical expertise, such as regarding the theoretical basis of a method, it is probably better asked there.