The constituant(e) is the person who gave the legal power to a representative to do things on his/her behalf (in this case her behalf, but her gender doesn't matter for translation purposes). If this were a last will and testament you would translate "constituant" in a specific way (settlor, testator...), but for a power of attorney you would use the more generic grantor (person granting a legal power).

In this case the grantor of the power is the seller. In order to sell her Moroccan real estate without having to physically go there herself, she gave her representative the power to do two things --

1. remettre tous titres et pièces (deliver all titles and documents), and
2. obliger la constituante à leur remise (create a legal obligation for the grantor to deliver them).

In order to complete a sale of real estate, certain titles and documents have to be delivered by the seller (or by someone acting on the seller's behalf). This power of attorney grants the seller's representative the power to deliver any titles and documents that they could possibly deliver, and also to do anything that would place the seller under a legal duty to deliver any necessary titles and documents that she may have.

For example, in a situation where the seller is doing all this herself, her signing an agreement of sale would place her under a legal duty to provide certain documents to the buyer (random example you might see in the US: documents proving that an addition that was built on the property was done legally -- i.e. copies of the construction permits, inspection reports, etc. -- so the buyer knows that the addition is legal and was competently built).

Here, she's granting a power of attorney to her representative; that means if her representative signs the agreement of sale, then she herself -- the seller -- is now legally obligated to provide those documents to the buyer.