From a simple perspective, a yoke is a means to harness a draft animal in order to pull a load.
However, a deeper understanding of this comes from considering it in an historical perspective. Back in Jesus‘ day, the learned Rabbi‘s would study the Scriptures. They did not see them as a definitive, fixed set of rules, but rather as a document to be explored and interpreted. A Rabbi’s interpretation of what the scriptures said, of what should be allowed and what should be prohibited, was called his yoke.
When Jesus said his yoke was easy and his burden light, he was letting it be known that he allowed many more things then he prohibited. He didn’t want his followers weighed down with a long list of don’ts, of heavy, burdensome requirements, but instead he wanted them to be free to focus on him — and not a bunch of rules.