This is the 5556th most frequent Russian word.
понятный
Clear; understandable.
'Понятными' here is an instrumental plural form of the adjective, agreeing with 'словами', illustrating the means of expression.
Научитесь писать понятными словами.
Learn to write in understandable words.
Here, 'понятное' is used as an adjective describing the subject 'объяснение', emphasizing its clarity.
Это объяснение очень понятное.
This explanation is very clear.
In this sentence, 'понятен' functions as a predicate adjective, relating to 'почерк' to describe its level of comprehensibility.