According to the dictionary, the definition is,

1. of or pertaining to worldly things or to things that are not regarded as religious, spiritual, or sacred; temporal: secular interests.
2. not pertaining to or connected with religion

So put simply, Secular Muslims, would be Muslims who perhaps have less regard for the Deen, and tend more towards worldly indulgences.

Or perhaps it could be interpreted, that secular muslims may be those who reject Ahadith, and the status of Sunnah, in favour of "modernising" Islam.

Allah Allmighty knows best