What were the sticks a sign of?
Rashi: They were a sign that Hashem had chosen Aharon as Kohen Gadol, and they would never again complain about the Kehunah. 1
Ramban #1: They were a sign that the tribe of Levi had been chosen over and above all the other tribes. 2
Ramban #2: They were a sign that the tribe of Levi had been chosen over and above the Bechoros. 3
Refer also to 17:17:1:1. Ramban: Perhaps Rashi too agrees that the sticks proved that Levi was the chosen tribe (Refer to 17:18:1:1), only, once Levi was chosen, it followed automatically that Aharon, who was the Nasi of the tribe, was the Kohen Gadol (See Ramban's objection to his own suggestion).
Ramban: As implied in Pasuk 23.
What are the connotations of the phrase "u'Sechal Telunosam?"
Seforno: It means that the sign of the sticks that are held as a safekeeping will put a stop to their complaints once and for all.