What is exactly is an agreement in Principal? If a developer has sold a piece of land and is proposing an agreement in principal which will disclose is contract with a third party prior to a group of investors removing a Les Pendens, is this a good idea? Does an agreement in principal protect both parties - the investors and the developer?

I don't understand the question. I have copied this from Wikipedia:
"Lis pendens is Latin for "suit pending." This may refer to any pending lawsuit or to a specific situation with a public notice of litigation that has been recorded in the same location where the title of real property has been recorded. This notice secures a plaintiff's claim on the property so that the sale, mortgage, or encumbrance of the property will not diminish plaintiff's rights to the property, should plaintiff prevail in its case. In some jurisdictions, when it is properly recorded, Lis pendens is considered constructive notice to the other litigants or other unrecorded or subordinate lienholders. Sometimes abbreviated as "lis pend"."
I am not familiar with the relationship between lis pendens and developers/investors.