Your question is an excellent one. While it is straightforward to explain the meaning of the sentence, it is quite difficult to explain the sentence gramatically. Austin and Lori have explained well the meaning. I will try to explain what I believe to be going on from a grammatical point of view. "Best", is both an adjective and an adverb. For example, "He is my best friend". (adjective) "As a member of the team, he swims best". (adverb) As an adverb, "best" is the superlative of "well", so you can say "As a member of the team, he swims well" to get a weaker statement. This cannot be done when "best" is used as an adjective. You cannot say "He is my well friend". You'd have to say "He is my good friend". So here's the mystery: in your sentence, "best" is clearly an adverb. BUT WHAT DOES IT MODIFY? It is tempting to say that it modifies "renewed". I think that conclusion would be false. The contract is not renewed well or best. We get a better idea what is going on if we rearrange the words like this: "It is best a contract renewed and renewed again." which has EXACTLY the same meaning as your original. When you write it this way, it becomes clear that "best" is an adverb modifying "is". Thus the thing that is "best" is not the renewing, but rather the manner of being.

It basically means "should be" or "needs to be". So, in this sentences: "that it’s a contract that should be renewed and renewed again" or "that it’s a contract that needs to be renewed and renewed again"

You’ve got it! In this example, synonyms for best are: optimally, ideally Every day you have a choice to be a good marriage partner and enjoy your marriage. You must renew your commitment daily.