In the last chapter, Paul talks about the big sin problem. So far, Paul addresses those who do such things (because sin ruins lives). But now Paul will give specific attention to those who judge others who do such things.
In particular, Paul proclaims that those who judge others "are inexcusable" because they "practice the same things" (2:1). Granted, there are instances where evaluating others is important (i.e.: voting), but often times, judging others is used for people to claim the moral high ground. Except judging others to claim the moral high ground does not work, and often times it creates a big hypocrisy problem! It's likely that the phrasing of "same things" is referring to the general sin category, so while we may not do the exact same sins as other people, we have stumbled in at least one area of sin.
Additionally, Paul talks about how "the judgment of God is according to truth" (2:2). Basically, it's impartiality, and because of that, Paul says that those who judge can not "escape the judgment of God" (2:3). So yeah, just because we judge other people doesn't mean we won't get judged by God.
It's also worth noting that man's judging strategies can differ from God's judging strategies by a large margin. Often times, man's judging is used to rashly throw down other people. But we know that God has "riches of His goodness, forbearance, and longsuffering", which "leads [people] to repentance" (2:4). In other words, God's strategy focuses on fostering repentance first (and Paul goes into greater detail in the next chapter, but spoiler alert, it involves Jesus's love). That doesn't mean that God won't deal with evil should people not repentant (especially people with a hardened heart), as we'll see in the next few verses.