Regarding C# syntax

Question

I do not understand the return statement, can anyone explain what ?, : does and explain about the return statement?
Thanks
public IAppTools Get(string appToolName)
{
if (string.IsNullOrEmpty(appToolName))
appToolName = Constants.APP_TOOL_NAME_DEFAULT;
return _appTools.ContainsKey(appToolName) ? _appTools[appToolName] : null;
}