Speed represents how far a character or creature can move in a single round. Speed is measured in feet and is determined by a creature's race and other factors like class features, spells, or items. For example, most humans have a base speed of 30 feet, which means they can move up to 30 feet on their turn if they use their movement action to do so.

Here's a breakdown of how Speed can be used:

1. Walking Speed: This is the most common form of movement. When you take the Move action on your turn, you can move up to your walking speed.
2. Running/Dashing: You can use your action to "Dash," which allows you to move up to your speed again. So if you have a speed of 30 feet, dashing would let you move another 30 feet on your turn.
3. Other Movement Types: Some characters and creatures have additional types of speed like swimming, flying, or burrowing speeds, which operate similarly to walking speed but apply to different types of movement.
4. Difficult Terrain: Certain surfaces like mud, snow, or thick undergrowth count as "difficult terrain." When you enter a square of difficult terrain, it costs an extra foot of movement for each foot you move.
5. Reduced Speed: Conditions like "Grappled" or "Restrained," as well as certain spells and abilities, can reduce your speed or even reduce it to zero, limiting your movement options.
6. Splitting Movement: You can split your movement during your turn. This means you could move 10 feet, take an action, and then move another 20 feet if you have a speed of 30 feet.
7. Interactions with Attacks: Some features allow you to make attacks while moving or immediately after moving a certain distance.

Speed is a vital aspect of combat and adventuring, affecting your ability to engage enemies, avoid threats, and navigate your environment effectively.