In general terms, the roles of these three branches are as follows. The legislative branch makes the laws. The executive branch carries out, or enforces, the laws that the legislative branch makes. The judicial branch interprets the laws when there are disputes about their meaning.
The legislative branch is in charge of making laws. Only it can do so. It is responsible for everything from deciding what the basic rules about school lunches are to deciding how much the US will spend on building a navy.
The executive branch carries out the laws. For example, Congress writes laws about taxes, but the Internal Revenue Service implements those laws. The executive branch is by far the biggest branch because most government employees, from park rangers to FBI agents, are part of this branch.
Sometimes, disputes arise as to what the law means. This is where the judicial branch comes in. It decides how to apply the law to specific cases that are in dispute.