To fetch the text you need this :
$("#data").find("input:checked").next("label").text();
     ^                ^                 ^        ^----------|
Where to look    What to find     Where to navigate    What to get
So basically, search in #data div for checked checkboxes and grab the
text of the label next to them.
Read :
$(function() {
  var prevText = $(".success").text();
  $("#submit-form").click(function() {
    var text = $("#data").find("input:checked").next("label").text();
    $(".success").text(prevText + " " + text).show();
  });
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<div id="data">
  <input type="checkbox" id="foo">
  <label for="foo"> Foo Text</label>
  <input type="checkbox" id="bar">
  <label for="bar"> Bar Text</label>
</div>
<p class="success" style="display: none;">Thank you for</p>
<button type="submit" id="submit-form" class="btn btn-success pull-left">SUBSCRIBE</button>
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solved Adding Text to Text if Condition is met