In informal language, when someone says something is "pretty simple" or "pretty good, or "pretty strong", the word "pretty" is used as an intensifier to emphasize the degree of simplicity or goodness. In this context, "pretty" is synonymous with "fairly" or "quite." It suggests that the thing being described is more than just simple or good—it's notably so. It's a way to express that something is more impressive or satisfactory than expected.