I think a post in a thread on The Straight Dope Message Board (Ten-hut??) explains it pretty well:
[*=1]a preparatory command, clearly given and somewhat drawn out, to prep the troops for what's coming next;
[*=1]a distinct pause, but not too long; and then
[*=1] a sharp, quickly-given command of execution so that the facing movement or marching order is executed together and the platoon (or squad, or detail or rank) moves as one.
As MarcB said, it's an order to come to attention - it's the word "attention" mispronounced in accordance with gramman's quoted explanation.
One day during the first week of boot camp, the RPOC (the recruit placed in a position of authority over all the other recruits in the company) said "Ten-HUT!" The company commander's response was "Don't bark, RPOC - Marines and other dogs bark." From then on the RPOC pronounced the word clearly: "Atten-TION!"