Are the gaps between the base molding and the flooring, or are the gaps because the base molding does not cover the necessary gap between the flooring and the wall?

Also, is the flooring prefinished? Floating snap/glue together flooring? Or unfinished when installed, sanded and finished in place?

Scribe the baseboard from the floor, use a plane, spokeshave, or other suitable tool to remove the variations. A bevel toward the wall side can ease getting to the contact line.
If you have flimsy, not too wide base, it can be sprung and nailed.

What kind of gap? A "vertical" gap -- between the bottom of the baseboard and the top of the flooring. Or a "horizontal" gap -- between the side of the baseboard and the edge of the flooring?

There needs to be room for expansion of the wood. If your flooring runs under the baseboard and the gap is "vertical" then you need to rehang the baseboard moulding, scribing as necessary. If the gap is horizontal, then the typical approach is to cover it with base shoe.

This is an on-site hardwood flooring. My contractor didn't do his work well when sanding so the floor is not equally flat. The floor goes under the baseboard.1/8" to 1/4" gaps are between the bottom of the baseboard and the top of the flooring.
Mike, What do you mean when saying rehang the base board?

Without a photo it's hard to say what to do, though alphonse's response is likely the correct one, if the unevenness isn't too bad.

If the sand job is very bad though, you might try to get the guy back there to re-do it. I can imagine the unevenness to be so severe or abrupt that a well scribed basebord will just excentuate it.

Try to Google "scribing wood trim" for an explanation; hopefully you'll also read about alphose's hint to bevel the edge to cut first. Scribing isn't a beginner skill, but it's not too difficult.

Bill

"My contractor didn't do his work well when sanding so the floor is not equally flat. "

Unless the floor was laid on TJI's or the like, stick framing is the likely culprit- lots of variation with grain direction and shrinking/drying, as well as possible installation error.

Even on new homes with factory-finished flooring, I end up using 1/8th round moulding. Only when the baseboard was stained oak on a stained oak floor could I forgo the moulding.

Depending on the colour contrast between the floor and baseboard, scribing may only highlight the variation.