Double Taxation Avoidance Agreement (DTAA) is a treaty signed between countries to avoid taxation at both the sides for a same good or service.
Let us take an example: You are having a Tea factory and you would like to export it to Singapore as there is a huge demand for it. Now if there is no agreement like DTAA between the country of your residence (that is India) and your exporting country (in this case Singapore), you need to pay export duty (read it as tax) in India and Import duty in Singapore. Now if you notice, you are paying tax in both the ends as a result your profit may come down. The point here is that, we prefer to export only when there is more profit to export than selling in our own market.This is considered as barrier for exports. So to promote exporters and importers of the both the countries, taxation is not done in one side and hence double the side taxation or double taxation in avoided. Though this an attempt to boost trade, it is misused especially with countries considered as tax havens.