For the most part, colleges treat the SAT essay score as if it were worthless. The only reason some colleges require it is because they want to make sure you can at least score above a certain threshold. As long as you can score higher than about a 4/4/4 our of 8/8/8/ you should be fine. Especially if you are applying to colleges that don't even require the essay. As for colleges that do require the essay, you can make up for the "low" essay score through writing good college application essays. If they see you can write a good college essay, then they definitely won't consider your SAT essay a weakness. Furthermore, I would like to add that the SAT essay score is really subjective right now. For the same essay, you may have received a completely different score depending on who graded it. This is why not much emphasis is put on SAT essay scores in general.
As to improving your essay, practice, practice, practice. I think the biggest setback when it comes to writing the essay is the fact that you only have 50 minutes to do so. Practice reading and writing 5-paragraph responses to articles under an hour. This should help bring your essay score up. Also, I suggest reading up on how the essays are actually scored: https://collegereadiness.collegeboard.org/sat/scores/understanding-scores/essay .
