The lesson is saying that the general rule is to take the feminine form of the adjective (ending in -a, e.g rápida, mágica, clara, ...) and add -mente, but if the adjective is the same for masculine and feminine, you just take that form, e.g feliz, legal, elegante... and add -mente to it.
rápidamente, mágicamente, claramente...
felizmente, legalmente, elegantemente... (these don't have a form for masculine and a form for feminine, they are the same, so "la mujer feliz, el hombre feliz" for example.
Hope this helps
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