Each player in a game of Warhammer 40,000 commands an army made up of units of models. You control all of the models in your army. If a rule refers to the 'controlling player', it is referring to the player who controls the models being affected by that rule.
A unit can contain one or more models. These models move and fight together as a single group. Most models have a base, which is also part of that model for all rules purposes.
Rules sometimes affect 'friendly' or 'enemy' models or units, which are defined as follows:
If a rule affects units or models without specifying that they are friendly or enemy, that rule affects any unit or model, regardless of whose army they are in. When an effect or ability applies to a unit, it applies to every model in that unit.
At any given time, one player is the 'active player' and their opponent is the 'opposing player'. Which player is which changes throughout the battle, but both players are always one or the other; whenever a player becomes the active player, their opponent becomes the opposing player, and vice versa.
While it is neither player's turn (e.g. at the start or end of the battle round), the player who takes the first turn in each battle round is the active player.
While it is a player's turn, that player is the active player, with the following exceptions:
Distances in Warhammer 40,000 are measured in inches (″). You can measure distances whenever you want to.
When a rule refers to a model's position in relation to anything else on the battlefield, unless otherwise stated, measure to or from the closest part of that model's base.
You require six-sided dice (often abbreviated to D6). There are many ways dice rolls are referred to, including:
To make a leadership roll for a unit, its controlling player rolls 2D6: if the result is equal to or greater than one or more of the Ld characteristics in that unit, that roll succeeds. Otherwise, that roll fails. The rule that instructed you to make that leadership roll will describe the effects of that roll succeeding or failing.
To make a battle-shock roll for a unit, its controlling player makes a leadership roll for it (see above).