The jQuery API .focus() method is used to make things happen when te user clicks inside a text box or textarea in a form.

You might want to use .focus() to have hints appear next to a text box so that the user enters the correct information or uses the correct format for the entry. Something like "This field is required" or "We will send a verification email to this address". 

$("input.alertField").val("");
$("input.alertField").focus(function(){
    $("td:last-child").hide();
    $(this).closest("td").next("td").show();
});

<table>
<tr>
<td>First name: </td><td><input type="text" id="LastName" class="alertField"></td><td> Enter legal first name</td></p>
</tr>
<tr>
<td>Last name: </td><td><input type="text" id="age" class="alertField"></td><td> Enter legal last name</td>
</tr>
<tr>
<td>Age: </td><td><input type="text" id="firstName" class="alertField"></td><td> Format: 35</td>
</tr>
</table>

The result: