Lithium-ion batteries experience the most chemical stress at the extremes, very close to 0% or very close to 100%. At these extremes, electrochemical strain inside the cell increases, which accelerates capacity loss over time.
Staying within the 20-80% range keeps the battery away from these high-stress zones and extends cycle life. Research shows a battery managed this way can retain noticeably higher capacity after a given number of cycles compared to one that is routinely run between 0% and 100%.
That said, this rule does not need to be followed with millimeter precision every single day; what matters is the overall pattern. Occasionally charging to 100%, before a long trip, for example, will not permanently harm the battery; the real issue is chronically keeping it at the extremes.
Most modern phones now offer features like 'optimized charging' or 'battery protection mode' that essentially automate this rule, learning your usage pattern and capping charge accordingly.
As a practical habit, charging to around 80% during daily use and plugging in before dropping below 20% is a simple routine that extends battery life without requiring any real extra effort.