Hi Shradha,
To use static resources in Lightning Application in salesforce, implement the application as follows:
<aura:application>
<link href=’/resource/bootstrap/’ rel=”stylesheet”/>
<div class=”navbar navbar-default navbar-static-top” role=”navigation”>
<div class=”container”>
<div class=”navbar-header”>
<a href=”#” class=”navbar-brand”>Lightning Contacts</a>
</div>
</div>
</div>
<div class=”container”>
<div class=”row”>
<div class=”col-sm-12″>
Contact list goes here
</div>
</div>
</div>
</aura:application>