If something is only marginally better, bigger, cheaper, or different, the change is so small it does not really matter. A score that is marginally higher is not a real jump. A plan that is marginally safer is still almost the same plan. This is a slightly formal adverb; in easy talk people often just say “a bit” or “only slightly.” Though actually, it also does the job of “barely / only just”: a shop that is marginally profitable is only just making money, sitting on the edge between yes and no. That “edge” picture is the link to margin and marginal. What this word does not mean, in ordinary use, is “in a way that pushes people aside.” That family of meanings belongs to marginalize and marginalization, even though they share the same first half.