Time dilation is caused by two main factors: relative velocity and gravitational fields. Both of these factors affect the passage of time, resulting in time dilation.
Relative Velocity: When two observers are moving relative to each other at significant speeds, they will experience time dilation. According to the theory of special relativity, time appears to pass slower for the moving observer compared to the stationary observer. This effect becomes more pronounced as the relative velocity approaches the speed of light. However, it's important to note that the difference in time experienced by observers at everyday speeds is incredibly small and not practically noticeable.
Gravitational Fields: In addition to relative velocity, gravitational fields also cause time dilation. According to the theory of general relativity, the presence of a massive object, such as a planet or a star, curves the surrounding spacetime. This curvature affects the flow of time, causing it to run slower in stronger gravitational fields. Therefore, an observer in a strong gravitational field, such as near a black hole or in the vicinity of a massive planet, will experience time passing more slowly compared to an observer in a weaker gravitational field.
Now, to address your question, time dilation can indeed be observed even at low speeds or while sitting still on Earth. However, the effects are extremely small and not practically noticeable in everyday situations. The gravitational time dilation on Earth is relatively weak, as the gravitational field here is not extremely strong compared to, say, near a black hole. Similarly, the time dilation due to our everyday speeds is negligible and requires much higher velocities to become significant.
In summary, time dilation is a real phenomenon caused by relative velocity and gravitational fields. While it is observed at high speeds or near gravity wells, it exists at all velocities and in all gravitational fields, but the effects are usually too small to be noticed in everyday situations.