This is the 5556th most frequent Russian word.
понятный
Clear; understandable.
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.
Его почерк совершенно понятен.
His handwriting is completely understandable.
'Понятными' here is an instrumental plural form of the adjective, agreeing with 'словами', illustrating the means of expression.