The trumpet plays a harmonic series, just exactly like a bugle. Starting on C on the ledger line below the staff, the series will sound as C, G, C, E, G, Bb, C, D, E with none of the valves depressed. If you depress the second valve, the trumpet will sound the exact same series one half-step lower. If you depress the first valve, one whole step lower. If you depress the third valve, one and a half-steps lower. Each valve opens up a length of tubing that is added to the length of the bugle, thus lowering the pitch. This is why depressing the first and second valves together works more or less the same as depressing the third valve. The whole thing is a little depressing, come to think of it.