Here's the breakdown:

* Rest Mass is Constant: An object's rest mass, which is its mass when it's at rest, is a fundamental property and does not change regardless of its location in the universe.

* Relativistic Mass Changes: However, in Einstein's theory of relativity, the concept of "relativistic mass" comes into play. This takes into account the object's velocity. As an object approaches the speed of light, its relativistic mass increases.

* Practical Implications: For everyday purposes, the difference between rest mass and relativistic mass is negligible. We typically deal with objects moving at speeds far less than the speed of light, so the change in mass is practically insignificant.

In summary:

* An object's rest mass is constant, no matter where it is.

* Relativistic mass increases as an object's velocity approaches the speed of light.

Let me know if you'd like to explore any of these concepts further!