Let us get this out first: YOU ARE NOT A NUMBER. Do not ever feel put down because of a grade or test score. Yes, numbers are important to the college admissions process, but do not let them decide where you apply or what you want to do with your life.
The most important grades are your junior and senior year grades. They show your most recent academic achievements. Your earlier grades will be viewed in relation to your most recent grades. It is best to show an improvement or steady achievement. You do not want to make the admissions committee think you are slacking off, and so it is best if your grades do not decrease.
Your grade will most likely be converted to a GPA as it provides the easiest way to compare various students from different schools. A GPA is based on a 4.0 scale and each school has its own conversion method. Schools will also have weighted and un-weighted grade point averages. Weight is given in the form of extra points or a certain percentage increase for Honors or AP classes. The most popular weighting is 104% of your grade for honors and 105% for AP classes. This is why it is very important to take proper difficulty of classes. Taking all easy classes, gives you a very low weighted GPA compared to students that take harder classes.