First of all, you are correct that it's "two" not "toot." It's part of the routine for a "Hand Salute" which is executed when the flag is raised or lowered. Here is a quote from the Marine Corps Drill and Ceremonies Manual (MCO P5060.20) that describes the procedure:
"1. The purpose of the hand salute is to demonstrate mutual respect and courtesy between members of military organizations and to show respect to national colors. It is executed in one count when halted at attention, marching at quick time, or seated in a vehicle. The command is "Hand, SALUTE." To return to the position of attention the command is "Ready, TWO."
"a. When "SALUTE" is given, raise your right hand smartly in the most direct manner until the tip of your forefinger touches the lower part of the headdress above and slightly right of your right eye. Your fingers should be extended straight and joined with the thumb along the forefinger. You should be able to see your entire palm when looking straight ahead. Your upper arm should be parallel with the deck with the elbow in line with the body and your forearm at a 45-degree angle. Your wrist and hand should be straight, a continuation of the line made by your forearm. At the same time, if not in ranks, turn your head and eyes toward the person or colors you are saluting.
"b. At the command "TWO," return to attention. Move your hand smartly in the most direct manner back to its normal position by your side.
"c. To ensure simultaneous execution of the second movement of the hand salute when troops are in formation, the preparatory command "Ready," will be used prior to the command of execution "TWO."
[end of quote]
The word "two" does not have any special symbolic meaning -- it's just the second part of the command sequence. Saluting is "one" and ending the salute is "two."
If the ceremony is accompanied by a bugle call or the playing of the national anthem, the "two" command is not required -- the troops execute the command automatically when the music ends. The MCO P5060.20 says:
On the last note of the “National Anthem” or “To the Color,” [or "Retreat" for lowering] all members of the detail terminate their hand salutes.
I hope that this answers your questions. Welcome to the forum!