If you would like to include many variants of a single word, i.e. different endings, you can use an asterisk = *. The asterisk represents no, one or several unknown characters. This is also sometimes called a wildcard.
migrat* = migrate, migrated, migrating, migration, migratory etc.
cat* = cat, cats, catlike, catty (but also catalogue, catharsis, catering etc!)
fish* = fish, fishing, fished, fisher etc.
Usually you don't have to use truncation to include the plural form of a word (it should be taken care of by the search tool), but to be on the safe side, we recommend you to do it anyway.
In some databases you may use the asterisk in the beginning of a word, for example *mycorrhiza* to include all of the following words: mycorrhiza, ectomycorrhiza, endomycorrhiza, mycorrhizae, mycorrhizas, ectomycorrhizae etc.
If you are uncertain of spelling or would like to simultaneously search for words that are similar but differ in the middle, you may use an asterisk or question mark in the middle of a word. For example, if you search for wom?n, you will simulataneously search for woman, women, womyn and womxn. Which character you use for masking depends on the database.
To find out how truncation works in your favorite database, check the following links:
In Google Scholar, truncation doesn't really work. However, Google scholar automatically searches for synonyms and word variants, but it covers far from everything.