Although soccer is a very fast paced sport with a running clock, there are many instances where the gameplay is temporarily stopped. This is a situation called a dead ball. Let's talk about what constitutes a dead ball in a soccer match.
The most common form of a dead ball is when the ball is kicked out of bounds and is no longer in play. In order to restart gameplay, a player stands out of bounds and performs an overhead throw-in. Once the ball crosses in bounds and is played by a player, the gameplay continues on until another dead ball is declared.
When a team makes a substitution, the ball is considered dead. The game restarts after a player makes contact with the ball in bounds, whether that be through a throw-in, corner kick or punt. When an injury occurs on the field, the official will signal that the gameplay is stopped and the ball is dead.
The most common thing for players to do when there is a dead ball while on the field is to stop moving entirely. This is especially common for injuries on the field, where players typically take a knee.
For other instances like throw-ins, players are not required to stand still on the field. They are often trying to avoid defenders and are moving around on the field awaiting the throw. For corner kicks and punts, players are usually paying extreme attention to the kicker and do not run until contact with the ball is made and the ball is deemed in play.
Think of it like this: when there is a dead ball, players cannot go after it.