A bam is a loud, startling sound. You might say that your brother tends to shove the front door open with a bam that makes you jump.
While a bam is a noise like a "bang," you can also use the word as an exclamation or interjection, to emphasize how loud or shocking something is. You might say, for example, "I was pulling out of my driveway, and then, bam, the truck hit me!" The word's origin is imitative — in other words, it sounds like a hit or slam. The first recorded use of bam, in 1917, imitated the sound of a military shell exploding.