When a human wants to become a lawyer, they have to go to school for many years to learn about laws and how to use them to help people. After they finish school, they have to take some hard tests to show that they really know what they learned. If they pass those tests, they can then become a "lawyer" officially.
But before they can start doing lawyer work, they have to go through one more important step called a "call to the bar". It's like a graduation ceremony for lawyers. Like how children have to graduate from kindergarten before they go to first grade.
During the "call to the bar", the lawyer stands in front of a group of important people called judges and other lawyers. They usually wear a special robe and hat, like Harry Potter. The lawyer says some important things and then they are officially welcomed into the "lawyer club". They are now allowed to do all the things that lawyers do, like represent people in court and give legal advice.
So when you hear that someone was "called to the bar", it means that they are now officially a real lawyer and can start helping people with legal problems.