what is html injection on a sample form having textbox, radio button, reset button, submit button code example

Example: what is html injection on a sample form having textbox, radio button, reset button, submit button

<form>
    <table>
        <tr>
            <td>
                <label for="uname">Name</label>
            </td>
            <td>
                <input type="text" id="uname" name="username">
            </td>
        </tr>
        
        <tr>
            <td>
                <label for="age">Age</label>
            </td>
            <td>
                <input type="text" name="userage" id="age" size="2" maxlength="2">
            </td>
        </tr>
        
        <tr>
            <td>
                <label>Gender</label>
            </td>
            <td>
                <label>
                    <input type="radio" value="f" name="gender"> Female</label>
                    <label>
                    <input value="m" type="radio" name="gender"> Male</label>
            </td>
        </tr>
        <tr>
            
            <td></td>
            <td>
                <input type="submit" value="Submit">
                <input type="reset">
            </td>
        </tr>
    </table>
</form>