A success roll is a die roll made when you need to test one of your skills or abilities. Sometimes you roll; sometimes the GM rolls for you. For instance, you might test, or roll against, your Strength to stop a heavy door from closing.
Roll 3 dice and add them together for a success roll. If your roll is less than or equal to the skill or ability you are testing, you succeeded. Otherwise, you failed. For example, if you are rolling against Strength, and your ST level is 12, a roll of 12 or less succeeds.
Thus, the higher the stat you are rolling against, the easier it is to make the roll.