URDF model not opening correctly on RViz or Gazebo.

edit

Dear all, I have the following URDF file:

<?xml version="1.0"?>
<robot name="r2d3">
    <link name="base_link">
        <visual>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
                <box size="0.5 0.5 1"/>
            </geometry>
            <material name="Cyan1">
                <color rgba="0 0.5 0.9 1.0"/>
            </material>
        </visual>
        <collision>
            <geometry>
                <box size="0.55 0.55 1.05"/>
            </geometry>
        </collision>
        <inertial>
            <mass value="1"/>
            <inertia ixx="0.5" ixy="0.0" ixz="0.0" iyy="0.5" iyz="0.0" izz="0.5"/>
        </inertial>
    </link>
    <link name="head_link">
        <visual>
            <origin xyz="0 0 0.5" rpy="0 0 0"/>
            <geometry>
                <sphere radius="0.25"/>
            </geometry>
            <material name="Cyan1">
                <color rgba="0 0.9 0.9 1.0"/>
            </material>
            <collision>
                <geometry>
                    <sphere radius="0.26"/>
                </geometry>
            </collision>
            <inertial>
                <mass value="0.5"/>
                <inertia ixx="0.5" ixy="0.0" ixz="0.0" iyy="0.5" iyz="0.0" izz="0.5"/>
            </inertial>
        </visual>
    </link>
    <link name="l_wheel_link">
        <visual>
            <origin xyz="0.25 0 -0.5" rpy="0 0 0"/>
            <geometry>
                <sphere radius="0.125"/>
            </geometry>
            <material name="red">
                <color rgba="0 0.2 0.5 1.0"/>
            </material>
            <collision>
                <geometry>
                    <sphere radius="0.130"/>
                </geometry>
            </collision>
            <inertial>
                <mass value="0.1"/>
                <inertia ixx="0.2" ixy="0.0" ixz="0.0" iyy="0.2" iyz="0.0" izz="0.2"/>
            </inertial>
        </visual>
    </link>
    <link name="r_wheel_link">
        <visual>
            <origin xyz="-0.25 0 -0.5" rpy="0 0 0"/>
            <geometry>
                <sphere radius="0.125"/>
            </geometry>
            <material name="red">
                <color rgba="0 0.2 0.5 1.0"/>
            </material>
            <collision>
                <geometry>
                    <sphere radius="0.130"/>
                </geometry>
            </collision>
            <inertial>
                <mass value="0.1"/>
                <inertia ixx="0.2" ixy="0.0" ixz="0.0" iyy="0.2" iyz="0.0" izz="0.2"/>
            </inertial>
        </visual>
    </link>
    <joint name="head_joint" type="continuous">
        <parent link="base_link"/>
        <child link="head_link"/>
        <origin xyz="0 0 0.5" rpy="0 0 0"/>
        <limit lower="-1.57" upper="1.57" effort="10" velocity="3"/>
    </joint>
    <joint name="l_wheel_joint" type="continuous">
        <parent link="base_link"/>
        <child link="l_wheel_link"/>
        <origin xyz="0 0.125 -0.5" rpy="0 0 0"/>
    </joint>
    <joint name="r_wheel_joint" type="continuous">
        <parent link="base_link"/>
        <child link="r_wheel_link"/>
        <origin xyz="0 -0.125 -0.5" rpy="0 0 0"/>
    </joint>
</robot>

The problem is that it doesn't open correctly on RViz. The error produced is: " No transform from [base_link] to [map] " This error is showing for every link in my file.

Some additional questions are: 1. What is the TF in URDF? 2. What is the Link Origin in contrast with the Joint Origin? (What does each of those represent?)

Thanks in advance.