One man's sin introduced the possibility of human death. However, one man provided a gift, and that gift "is not like the offense" (5:15).
Paul talked abut how "by the one man's offense many died" (5:15). Due to the introduction of death from sin, many people who have done some not-so-good things unfortunately also get affected. Adam (and Eve) instigated the opening; we reaped the result. But the other one -- Jesus Christ -- paved the way for "the gift by the grace", which "abounded to many" (5:15). Jesus's gift of His righteousness, peace, etc. is offered for everyone -- and many people who believed in Jesus get to experience His gifts.
Additionally, Paul talked about how "the judgment which came from one offense resulted in condemnation" (5:16). Considering that God has high standards and does not like sin ruining lives, all it takes is one bad action to result in a bad record. But in contrast, "the free gift which came from many offenses resulted in justification" (5:16). There are multiple senses in this quote. First off, Jesus's gift is available even for those who screwed up badly (and need Jesus for recovery). Second, part of the reason why Jesus's crucifixion happened is due to the offenses of many different people -- Romans, Jewish leaders, etc. Some people even talked about how on the cross, Jesus bore the weight of our sin. And as we saw in Romans 5:9, we are justified by His blood (where His blood fulfills Hebrew law, marks the start of the New Covenant, declares Jesus worthy to offer forgiveness, etc.).
Finally, Paul talked about how "by the one man's offense death reigned through the one", but "those who receive abundance of grace an of the gift of righteousness will reign in life through [...] Jesus Christ" (5:17). In other words, Jesus Christ countered the power of death. He overpowered death via the resurrection, and he offers eternal life (with the resurrection) for anyone (even YOU) who believes in Jesus Christ as Lord and Savior.