You get this exception mostly with form submissions. Laravel protects application from CSRF and validates every request and ensures the request originated from within the application. This validation is done using a token. If this token mismatches this exception is generated.

Quick Fix

Add this within your form element. This sends csrf_token generated by laravel along with other form data so laravel knows that your request is valid

<input type="hidden" name="_token" value="{{ csrf_token() }}">