If you get arrested, you are only going to get one chance to prove your innocence in court. Most people hire a lawyer to aid in their defense, but others spurn legal counsel in lieu of being their own attorney. In any case, having the correct knowledge of legal terms is imperative to providing a solid legal defense.
If you walk into the courtroom ignorant on law terminology, not only will your weakness be exploited, your credibility will go right out the window. Have no doubts about it, the prosecution or defense will exploit the fact that you probably do not know most law terms.
Some people may not say that is important, and that the important part of a trial is simply providing a competent and reasonable defense. The importance of legal jargon depends on whether you are in civil court or criminal court as well.
When two people are in civil court, neither person is looking at a jail sentence as the outcome of the case, so law terms are not as important. Legal terminology is important in that both parties have to know what is going on in the case. For example, it is important to know what it means to be held in contempt, or what a recess is, but if the prosecutor asks you to make an avowal and you appear stumped, it is probably not the end of the world. For those who do not know law terms, an avowal is just a declarative statement.
By and large, one can provide ones own legal defense adequately, if and only if one truly studies at very least the basics of law, and especially if one is actually innocent. At the end of the day however, it is probably in the best interests of the defendant to enlist the services of a professional, trained attorney at law.