Edge Of A Sumo Ring
Last Moment, Critical Moment
A soil sack on the edge. Pretend the sacks are lined up around in a circle. It's the edge of a sumo ring. It can also refer to a last moment or critical moment in time, like the moment before getting pushed out of the ring.
The first two kanji (土俵) use the on'yomi reading you learned. The last kanji 際 uses the kun'yomi reading, which you should know via the vocabulary. Just think of these two parts as somewhat separate bits of the word, which is why one part is on'yomi and another (the addition) is kun'yomi.
The sumo wrestler braced himself at the edge of the ring and was able to throw his opponent out.
The peace negotiations ran into trouble at the last moment and were broken off.