Ligatures are special characters that tie two or more other characters together to fix spacing problems.
The most common ligatures are designed for these letter combinations.
ff fi fl ffi ffl fh fj fk
But there may be many more depending on the typeface.
Not all fonts have ligatures.
Not all fonts need ligatures.
If a font has ligatures, the software does all the work. Easily toggle “ligatures” on and off in the Adobe products.