He is making arrangements for his wedding to Juliet, just as Romeo did in Act 2.
she says truthful things to hom that he understands
Paris is there, arranging his wedding to Juliet.
When Juliet arrives at Friar Laurence's cell, it is Paris who is speaking with the Friar. Paris is there to discuss marrying Juliet, but Friar Laurence is aware of her secret marriage to Romeo. This sets up the conflict that will lead to the tragic events of the play.
The message from Romeo is to arrange for Juliet to come to Friar Lawrence's cell for their wedding ceremony.
The first time when she arrives at his cell, Romeo is there ready to get married to her. The second time Paris is there, making arrangements to get married to her. There's a kind of parallelism there.
In a previous scene, Capulet and Paris agreed that Paris should marry Juliet on Thursday at St. Peter's Church. However, it is the groom's job to make the arrangements with the priest, which is what he was doing at Friar Lawrence's cell. Romeo was the one to make arrangements with Friar Lawrence about his wedding to Juliet, as you will remember.
Killing himself
Juliet goes to Friar Laurence's cell in the play twice. The first is to marry Romeo and the second is after she learns of her prospective marriage to the County Paris. The latter time she seeks his help to stop the marriage from happening so that she may remain faithful to her Romeo and bee reunited with him.
Friar's Cell
Juliet goes to Friar Laurence's cell to seek his help and advice after being told by her parents that she must marry Paris. She is desperate for a solution to avoid the unwanted marriage and turns to the friar for guidance.
He gives her the Sleeping Potion to simulate death, so that she can avoid marrying Paris and committing bigamy.
To meet him so they can arrange how she will escape the planned marriage. They meet the priest who gives her the sleeping potion.