If the Find As You Type option is selected, entering/deleting characters in the Find edit field results in automatic full-text searching for the whole table.

If the Find In Current Results or Find Similar In Current Results options are selected, searching is performed within the current record set only, and using the subsequent Find All command in these modes performs an AND search, progressively narrowing the search results.
The Find Similar option matches strings the same way spell checking does, to find suggestions for misspelled words.

By default, the Find As You Type mode and the Find All command in the other modes enable searching all fields. Making any selection that spans one or more columns/fields will limit searching to those columns/fields; for example, selecting a range of 1 row x 3 columns will result in searching within those 3 columns/fields only.