That's because MATLAB does arithmetic with double-precision numbers, which are only accurate to around 15 decimal places. Of course, but because of round-off error, MATLAB has given the numerical answer (in scientific notation). MATLAB has recognized sin as referring to the sine function, and pi as referring to. This enables you to use MATLAB as a calculator. Position your mouse there, type something, and hit Enter (or Return). The most important of these is the Command Window, where you should see a prompt symbol that looks like >. How MATLAB appears when it first launches depends to some extent on what kind of computer you are using and what defaults you have set, but in all cases you should see the MATLAB Desktop with a number of subwindows in it.
