According to Einstein's theory of relativity, there will indeed be a time difference between the two observers' clocks when they meet again after one of them has traveled at a significant fraction of the speed of light.
This phenomenon is known as time dilation, and it arises from the principles of special relativity. According to this theory, time is not absolute but rather depends on the relative motion between observers. When an observer moves at high speeds relative to another observer, time appears to pass slower for the moving observer from the perspective of the stationary observer.
In the scenario you described, let's say Observer A remains stationary while Observer B travels away from A and then returns. From the perspective of Observer A, time will pass at a "normal" rate. However, from the perspective of Observer B, who is moving at a significant fraction of the speed of light, time will appear to pass slower.
As a result, when they meet again, Observer B's clock will show less elapsed time compared to Observer A's clock. The magnitude of this time difference will depend on the speed at which Observer B traveled and the duration of their journey. The faster the speed of travel, the greater the time dilation effect.
It's worth noting that the time difference observed by the two observers is entirely reciprocal. From Observer B's perspective, it would appear as if time passed more quickly for Observer A while they were moving away and then slowed down when they turned back. This symmetry is a fundamental aspect of relativity theory.