In doing a calculation with
Mathematica, you usually go through a sequence of steps. If you want to, you can do each step on a separate line. Often, however, you will find it convenient to put several steps on the same line. You can do this simply by separating the pieces of input you want to give with semicolons.
If you end your input with a semicolon, it is as if you are giving a sequence of operations, with an "empty" one at the end. This has the effect of making
Mathematica perform the operations you specify, but display no output.