If you know the angles, you know the position
Forward kinematics answers a straightforward question: given the angle of every joint in a robotic arm, where does the tip of the arm, the end effector, end up in space? Imagine a simple two-joint arm bolted to a table, like a shoulder joint and an elbow joint. If you know exactly how far each joint has rotated, you can calculate the end effector's exact x, y, z position using basic geometry, treating each arm segment as a rigid rod at a known angle.
This calculation is deterministic and always has exactly one answer. Set the shoulder to 30 degrees and the elbow to 45 degrees, and there is one, and only one, position the end effector will end up at. That predictability is what makes forward kinematics the easier of the two problems this lesson covers.