The phenomenon you are referring to is called time dilation, and it is a fundamental consequence of Einstein's theory of special relativity. According to this theory, as an object with mass (like a spaceship) approaches the speed of light, its relative velocity with respect to an observer at rest will cause time to pass differently for the moving object compared to the stationary observer.
The faster an object moves, the slower time appears to pass for it from the perspective of the stationary observer. Conversely, from the perspective of the moving object, time appears to pass normally. This effect becomes more significant as the object's velocity approaches the speed of light.
To explain this, let's imagine a scenario where a spaceship travels at an incredibly high speed, very close to the speed of light. For simplicity, let's assume it can reach 99.99% of the speed of light. From the perspective of the astronauts inside the spaceship, they would experience time just like they always do, unaffected by their high speed.
However, from the perspective of an observer on Earth, watching the spaceship whiz by, time onboard the spaceship would seem to slow down. This means that the astronauts' journey would appear to take much longer to the observer on Earth compared to how long the astronauts themselves perceive it.
The apparent "push back" you mentioned arises from the fact that as the spaceship tries to reach the speed of light (which is not achievable by massive objects due to the infinite amount of energy required), the time dilation effect becomes more extreme. Time dilation approaches infinity as you approach the speed of light. Consequently, it would require an infinite amount of energy to accelerate a massive object to the speed of light, which is why we consider it unattainable.
In summary, the "push back" or apparent resistance when trying to travel at the speed of light or faster is a consequence of time dilation and the fundamental limitations imposed by the theory of special relativity. As an object with mass approaches the speed of light, the energy required to accelerate it further increases dramatically, making it practically impossible to reach or surpass the speed of light.