Two different keys landing in the same bucket is a hash collision. It happens because a hash function squeezes a huge space of possible keys into a small, finite number of buckets.
There are far more possible keys than slots, so by pigeonhole some keys must share a slot. Even a great hash function cannot avoid this; it can only make collisions rare and evenly spread. Poor functions make it worse by clustering keys together.
Collisions are normal, not a bug, so every hash table has a plan for them. Common approaches store the clashing entries in a small list per bucket, or probe for the next open slot. Handling them well is what keeps lookups near O(1) instead of degrading.
Rewriting in plainer words…
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