METT-TC is an acronym used by the US Army to help commanders remember and prioritize what to analyze during the planning phase of any operation. It stands for Mission, Enemy, Terrain, Troops available, Time, and Civilian considerations.
The mission element of METT-TC is basically the consideration of what the mission is at that given moment. Depending on the type of mission, you may be allowed more or less operational freedom to make decisions.
The enemy element of METT-TC is of course the application of the expected enemy force (both size and type) in which you are likely to make contact with. By knowing this piece of the puzzle, you can make better decisions on how you might move, actions on contact, and what to engage with.
The terrain element of METT-TC is the adherence to the terrain conditions (including weather and time of day) and the terrain type in the area that the mission is being carried out on. This may effect the selection of formation type when moving, the movement speed, the visibility, as well as tactically advantageous positions located on high ground. When moving, it also determines the route of travel where, in almost every case, movement should be made "as water flows" (in the low ground) for greater security.
The troops element of METT-TC is the knowledge of the composition (size and type) of friendly forces in the area that can be utilized for the mission. Knowledge of this piece of the puzzle can allow for efficient application of force as well as being able to bring sufficient force and support to bear to deal with any enemy threat.
The time element of METT-TC is knowing how much time is available to complete the mission as well as knowing the time and place to coordinate actions. Time is vital in the synchronization of both tactical and strategic missions, to be on time to make an attack that is coordinated with another, or to move in coordination with another friendly movement. Time considerations also sets the tempo of a mission.
The civilian considerations of METT-TC involves knowing where civilians are located and usually involve the restrictive use of force so as to minimize civilian casualties and collateral damage. For example, the civilian consideration would play a role when determining the magnitude of force to be used in eliminating an enemy that is located in an urban environment.