Pitch refers to the frequency of a sound, while loudness refers to the size or amplitude of the sound. Every sound has both a specific pitch and loudness.
In order to simplify pitches, people assign them letter values. For example, the pitch of the piano key "middle C" is 262.63 hertz, a unit of frequency. Humans can only hear frequencies in the 20 to 20,000 hertz range, while other animals have different hearing frequency ranges.
The loudness of a sound will also impact how easy it is for a person to hear it. A loud noise, one with a large amplitude, will be much louder and easier to hear.