what is merihari?

what is メリハリ really means ..how to use it ?

"merihari no aru" is the opposite of "monotonous" and "boring (because of lack of changes in pace, tone, etc.), like if you were playing a tune on your piano, the tone should not be dull and monotonous, but you should play it with a lot of gusto where that is needed, and quiet and elegantly where that is needed, so giving it a good contrast.

Actually it's one of the words that I have difficulty translating into a simple English word/phrase.

It depends on the context. Is this financial? Merihari wo tsuke is a bit difficult to translate but means to reallocate (a budget) money from low to high priority items. Of course, I am implying that there is more to the sentence than just merihari.
In a musical sense, I like AK's translation. Can also be merihari no kiita and a bunch of other forms.

BTW, as the etymology, I heard that "meri" and "hari" means "low pitch" and "high pitch" in the traditional Japanese music.

Just some examples:

- Haru yasumi wa, merihari wo tsuketa seikatsu wo shite kudasai. (teacher saying to school children before the spring break: "During the spring break, live with proper "merihari" - meaning, do not go lax all the way and sleep and goof off all the days, but study when you do, play when you do, give yourselves a good rhythm/accents in your life)

- Baraado kara dansu ongaku made, merihari ga kiita, ii konsaato datta. (From ballad songs to dance music, it was a good concert with good accents.)

- Kimi no purezen ni wa, merihari ga nai. Nani ga iitai'n da? (boss saying to his subordinate: Your presentation lacks proper accents. What do you want to say?)

"Merihari wo tsukenasai (give it some "merihari")," or "merihari ga nai (you don't have merihari)" are phrases often used to scold when things get too lax or monotonous :)

One context in which I came across merihari, was in reference to driving instructors at a certain examine center preferring merihari. Maybe it would be useful to translate it as variation, variation in pace, tone, or pitch?

Just an additional question: is it always written in Katakana or can you use Kanji?

Merihari is commonly written in Katakana or Hiragana.
Now I googled and knew that merihari is 減り張り in Kanji, it's a bit hard to read, I think.