Alpha-linolenic acid, or ALA, is a poly-unsaturated fatty acid (or PUFA) found in plants such as soybean, flaxseed, and rapeseed (canola). It is one of the three main omega-3 fatty acids, the other two being DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid), which are synthesized by microalgae and, because fish and crustaceans eat phytoplankton which have a diet consisting of microalgae, also end up in fish and seafood. 

ALA consists of a chain of 18 carbon atoms. Poly-unsaturated fatty acids are characterized by the fact that they have double (=unsaturated) bonds between carbon atoms in multiple ("poly" in Latin) locations in the fatty acid chain. ALA has three such double bonds. The term "omega-3" is derived from the fact that omega is the last letter in the greek alphabet and the first double bond between carbon atoms in ALA is located 3 carbon atoms away from the tail end of the chain, hence omega-3. Sometimes omega-3 is referred to as n-3 and since ALA has 18 carbon atoms, 3 double bonds and is an omega-3 fatty acid, its shorthand-code is C18:3n-3. 

ALA is also a so-called essential fatty acid, because the human body does not synthesize ALA and therefore people need to get it from their diet.