追 (zhuī): To chase or pursue. Imagine desperately running after something that's already gone.
悔 (huǐ): To regret or repent. This is the core feeling of sorrow for a past mistake.
莫 (mò): A classical and formal negative particle meaning “do not” or “impossible to.” It's stronger and more literary than the common 不 (bù).
及 (jí): To reach or catch up with.
Combining them literally gives you: “To chase the regret, but find it impossible to catch up.” This vivid imagery paints a picture of someone trying to undo a mistake, but the consequences are already too far ahead to be changed. You can regret all you want, but you can't reach the past to fix it.