It will depend in the company, but in generally for internships/first jobs grades tend to be an important factor as there's little else distinguishing between most candidates.
Some companies might have minimum grade policies which means you'll get automatically filtered out. In some cases you can get around this by going a less conventional route (i.e. speaking to an employee and getting them to let you bypass the filter).
Most of the research in recruitment studies show that early impressions stick, so once they've decided "we (don't) want to interview this person" they'll likely stick with that position regardless of future information.
Hence for companies which don't pre-screen your best chance is making sure they see what makes you stand-out (personal projects, experience, etc.) before they see your grades. So make sure your CV skews in that direction.