By default, you'll merge on a language, making sure all translations for a word in the source are merged. But you can also select an entry field and make sure all entries with the same filed value are merged. Now, what would that be good for? It can be used to preserve homonymns. You need a field that uniquely defines a concept, even if the same word is used. 

If you have that, you can export your termbase, make some changes, merge the old and the new termbase, and the changes get added to the correct entry. The example below mixes financial and tourist data, with homonyms and synonyms, but thanks to entry numbers gets everything right during merging. They are really useful, but need diligent maintenance. The easiest way is to start from an sdltb file: they have a built in Entry number field. This is exported with the each entry when converting from sdltb. 

When you want to use a merge field, make sure you actually filled in the field name in the settings! It's easy to forget, in which case the existing setting, usually a language, is used. I speak from repeated frustrating experience...

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