Drill is marching, plain and simple. In ancient times, the most powerful, efficient and developed empires developed ways of moving troops from one place to another without them getting mixed up with other troops. The theory was, without drill, masses of soldiers would end up getting lost on the way to battle, and have to fight with just any old unit they could find, instead of the unit they trained with. As time went on, a system of flags was developed which allowed soldiers to find their own units (and side) on the battlefield if they got lost. However, the military quickly discovered that sticking to "formed up" units worked better, as everyone was present when needed for battle. Overall, the drill system worked: soldiers stayed together and could be commanded as a group.
These days, military drill is mostly used for military ceremonies, such as military parades, and to instill pride and discipline during military training (such as basic training). In fact, the military drill manual (a book about six inches thick), confidently states that "drill is the foundation of discipline in battle," and that "this has been proven again and again." For example, back in 2002 and 2003, when the North Koreans claimed that they were developing nuclear weapons, it conducted a huge number of public military parades, and they received lots of attention from the press, who commended on how "modern" and "disciplined" their military was. They could clearly be seen to be skilled; and this would act as a deterrent against many lesser military forces.
For today's military, regular parades in public displays the military as a highly trained, disciplined and professional, military force. Even though the parade itself does not provide any useful function on the battlefield, it instills public confidence in our military forces. A rag-tag military is likely to be unable to put on parades, hence it holds that larger and better military forces can display their discipline by means of public performances.
There are two basic types of drill: stationary drill and marching drill. Stationary drill is just what it sounds like – drill while not moving (marching). Marching refers to drill movement while actually moving (marching) in formation. Stationary drill are drill movements that are accomplished without marching. The drill positions of “attention” and “parade rest” are two perfect examples. Once you start getting the hang of stationary drill commands, your training instructor will take your unit to the next level, and have you start marching (you march everywhere you go in basic training).
When performing marching drill, both the preparatory command and the command of execution are given as the foot in the direction of the turn strikes the ground. For single formations, the preparatory command is normally given as the heel of the left (right) foot strikes the ground, and the command of execution is given when the heel of the left (right) foot next strikes the ground. For multiple units, time is allowed for the subordinate commanders to give appropriate supplementary commands. The pause between commands is three paces. To march forward from a halt, the command is "Forward, March." This is just one of the many specific commands given to lead the marching group, and they can get somewhat complex and sometimes confusing. There are also very specific ways to move your body - in inches and body formations - in order to create a seamless, organized drill group. But trust me - - as time marches on, you will get in some good practice time with your group at boot camp/basic combat training. Military drill will then become a second nature, and a ceremonial tradition of the military sure to bring a smile on your face (when commanded).