You can use the title attribute on an i element, like any element, e.g.
<i class="icon-lock" title="This symbolizes your being locked inside"></i>
Whether it helps is a more difficult issue. Browsers usually show the title attribute value as a “tooltip” on mouseover, but why would the user mouse over the icon? And such tooltips are of poor usability; so-called CSS tooltips often work better.
Screen readers may give the user optional access to title attributes, but I’m not sure what they do with elements with empty content.