What does it mean when a conference submission system asks if the first author is a student?

It can mean lots of things and only the conference organizers can tell you the reason. There could be student paper awards, travel awards, or student presentation sessions. There could be research speed dating or a desire to have more student presentations in each section.

Likely the peer review will not change, but the selection process might chnage. Paper selection is not just about the best paper, but about the best fit given the goals of the conference.

As suggested in @Distic's comment, the purpose of this question is probably to identify candidates for a student paper award.

does this means the reviewers will consider this and will be less strict?

In a decent conference, probably not. The audiences of these conferences expect that high standards are applied to submissions. These standards need to be enforced regardless of the authors' identities.