In a word list, a key word list or a list of collocates you may want to store several entries together: e.g. want; wants; wanting; wanted. Bringing them together means you're treating them as members of the same "lemma" or set -- rather like a headword in a dictionary.

 

A lemmatised head entry has a coloured mark in the left margin beside it. The others you marked will be coloured as if deleted. The linked entries which have been joined to the head can be seen at the right and in the status bar below.

 

 

You'll see n/a in some columns because if you join after a word list was computed some information is no longer available. Fix it with a lemma consistency check.  

Choose View | Show joined items to see a list of the lemma forms, or simply double-click the Lemmas column.

 

See also: joining words, manual joining, auto-joining, choosing lemma file