Close up filters and Teleconverters are two different things altogether. Close up filters are screw on filters added to the front of the lens to enable your lens to be able to focus on close-up objects, for example flowers and insects. Close up filters are primarily used for macro photography. Teleconverters on the other hand extend your working distances from the subject (usually by a factor of 1.4x or 2x) so the focal length of your lens also increases proportionately. For e.g, your 75-300mm lens becomes an effective 150mm-600mm lens if a 2x teleconverter is added to it.
