In the sentence, 'wash' means 'to be able to be washed without losing colour or being damaged'.
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t0mWiNn31I like this shirt because it washes well.
It is easy to clean and it does not look rumpled or old after you wash it. It looks fresh and new.
t0mWiNn31 Active & Passive
[ I like this shirt ] 1 because [ it washes well ] 2.
Both clauses 1 and 2 can be called active, but in some of the linguistics literature you will see that 2 is said to be in the "middle voice", neither active nor passive, because the shirt ("it") is not really an agent capable of washing anything.