Chances of success in Hero System
Hero System uses a 3d6 roll to determine success. You try to roll below your skill level (plus any modifiers) or less.
But a 3d6 roll has a bell-curve distribution, which means modifiers affect effective success chances rather differently to resultion systems like d20 or d100, which have flat distribution curves. In a d20 system, a +2 modifier is always equivalent to a 10% greater chance of success - but in Hero System (or GURPS, which also uses a 3d6 resolution system), it varies depending on the starting success chance. The effect of modifiers is greater the closer the target number is to 10 or 11, and has a decreasing effect the further away from this average the starting success chance is.
Despite this, it's pretty easy to calculate the chances of success once base skill level and any modifiers are taken into account. Let's call base + modifiers the Effective Skill Level.
An effective roll of 18- would be a 100% success chance, but in Hero System, a roll of 18 always fails (and a roll of 3 always succeeds) no matter what the effective skill level, so you never have a success chance greater than 99.53%.
Success in Combat
Hero System uses a simple comparative method for hitting things. The basic success roll in combat is 11 or lower, but this is modified by bot hthe attacker's offensive combat value (OCV) and the defender's defensive combat value (DCV), to the final target number is 11 + OCV - DCV (Hero System 6e describes this slightly differently, but the formula remains the same).
As a result, it's crucial to balance values for Combat Values properly in Hero System. It matters little how high the values are, so long as they're well balanced to the challenges PCs are likely to face. Because this is Hero System, it's not too bad to present weaker opponents in the early stages of a mission, so long as significant fights are challenging.
Hero System uses 3d6 rolls, which follow a bell curve distribution, so an an advantage of a point or two can make a significant difference to whether a character succeeds or fails in an attack.