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 red 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.
Here we see a word list based on 3-word clusters where originally a good deal had a frequency of 24, but has been joined to a great deal and a good few and thereby risen to 141.
How to do it
The menu offers choices for joining and un-joining.
View all the various lemma forms
Double-click on the Lemmas column as in the shot below,
and a window of Lemma Forms will open up, showing the various components.