The 50/30/20 rule is a simple way to split your monthly net income into three big buckets: 50% for needs (housing, transport, basic food), 30% for wants (leisure, travel, subscriptions) and 20% for savings or debt repayment beyond the minimum.
It is not an absolute truth: in expensive cities 50% on needs is nearly impossible, and on low salaries 20% savings is too. It works as a starting point to see where your money goes and decide which bucket to adjust.
To apply it sensibly: start from net income, not gross. And review every three months whether the percentages still match your real life.