Short answer
The load factor (default 0.75) is the fullness threshold: when the number of entries exceeds capacity * loadFactor, the HashMap resizes -- it doubles the bucket array and rehashes every entry into the new, larger array.
Explanation
Keeping the table under ~75% full keeps chains short so lookups stay near O(1). Resizing is expensive but happens rarely, so its cost is amortized across many inserts; 0.75 is the standard time/space compromise.
Common mistakes
- Saying the load factor is the maximum number of entries.
- Not knowing the default 0.75 and the resize to double capacity.