An unreserved (non-reserved) seat — seating not allocated in advance, available on a first-come, first-served basis.
新幹線の自由席は朝は混雑している。
The unreserved cars on the Shinkansen are crowded in the morning.
コンサートは自由席と指定席に分かれている。
The concert has both unreserved and reserved seating.
切符は自由席用を買ったので、好きな場所に座った。
I bought a ticket for unreserved seating, so I sat wherever I liked.