Checkbox
A checkbox can hold two states - checked or unchecked.
The Metafield value of a checked checkbox (!) is represented as a string value of “true” but an unchecked checkbox will cause the underlying Metafield to be deleted rather than contain a value of “false”.
Accordingly, we can test for it being checked or not this way:
{% if product.metafields.accentuate.washable %} <p>{{ product.title }} can be washed</p>{% else %} <p>{{ product.title }} can NOT be washed</p>{% endif %}