What does a squeaky sound coming from the left side of car when driving mean?

It USUALLY means you need new front disc brake pads; there is a "squeaker" on the brake pad backing that contacts the disc intermittently. When you apply the brakes, if the squeaking goes away, or becomes quieter, that is what you need to replace. Most disc brake pad sets run $13 - $40 per set (right and left). If the squeak is always on, you need to see the mechanic. If you just need pads, any auto mechanic student you may know can do a fine job for you real cheap!