Hefker is a legal status which means that an item has no owners and that anyone is free to claim it. My daughters nursery teacher asked me if I agreed that any property left behind at the end of term be declared hefker. This was to save the teacher from chasing up the owner of such insignificant items such as nappy wipes, which otherwise she would have been legally obligated to do.
Hefker comes from the verb pakar, which in Mishnaic Hebrew means to set free. Any object declared hefker is set free of its owners.
During the shmittah (sabbatical) year, beginning this Rosh Hashanah, when farming is prohibited, all produce is hefker and people are free to enter anyones fields and help themselves to whatever has managed to grow without human assistance.
Outside of the legal context, a state of hefker, known as hefkerut (or in Yinglish, hefkeyrus), means lawlessness or neglect, as in, What some parents would call giving children their independence, others would call hefkeyrus. Hefker-mentsch is Yiddish for a derelict person.
Hayim Nahman Bialik wrote about the source of his poetic inspiration, lo zachiti baor min hahefker, I did not gain from the light of hefker; Bialik is claiming that his poetry is not copied from foreign styles and sources that are lying around, hefker for anyone to use.