Pitch is how high or low a note is.
To get more technical: If sound means the vibration of air, then for every sound, the air must be vibrating at a certain frequency. How frequently is the air vibrating? In other words, how many vibrations per second are there? That’s pitch. In this way, pitch can be measured literally in vibrations per second, called Hertz (named after German physicist Heinrich Hertz, for reasons you can look up). An example would be 440 Hz, which is a medium-high note anyone with normal hearing can hear.
To get more practical: Since saying “440 Hz” is hard to say fast, we just say “a,” which means the same thing. (At least, this is the standard tuning for a. Other tunings are rare and hardly worth mentioning, but now you know they exist. You should always assume a = 440 Hz.)
To get even more practical: There’s no pressing need to be aware of the frequencies of notes we use in music. It’s quite enough to know about the musical alphabet, which gives us 7 letter-names (a through g) and 5 sharps and flats, for a total of 12 notes. Aren’t there more than 12 notes though? Of course, but they all have one of these 12 names. The only difference might be the octave, which we’ll get to later.
Not all instruments require pitch. Drums are generally used predominantly for rhythm, often without the need for any specific pitches. Have you ever heard a cymbal crash? Would you say it has an actual, specific “pitch”? I would hope not. It might be “high,” but we don’t care exactly how high it is. Therefore, we don’t bother to give it a letter-name.
Quick and dirty rule:
Percussion (drums) = unpitched;
everything else (singers, strings, brass, woodwinds, keyboards, including the piano) = pitched.
Hilarious matter of confusion: The piano is technically a percussion instrument, since its hammers strike its strings. But clearly, the piano produces specific pitches: That’s what all those keys are for! So the piano is a pitched percussion instrument. (I did say the rule was “dirty.”)
Alright, I’ll go ahead and clear this up: Except for odd circumstances, the only type of instrument that can be unpitched is a percussion instrument.
But plenty of percussion instruments are pitched: xylophones, vibraphones, glockenspiels, marimbas, steel drums, etc.
How about unpitched instruments? Think of anything that sounds more like . . . a sound . . . than a note: cymbals, tambourines, sleigh bells, shakers, triangles, snare drums, bass drums, rain sticks, djembes, etc. (Look any of them up if you want.)
Finally, there are a few (usually percussion) instruments that fall somewhere in the middle: They’re sort of pitched, but sort of unpitched: Tom-tom drums are a great example. We want to hear “high,” “medium,” and “low” toms, but we don’t necessarily care what pitches they’re tuned to. Likewise, woodblocks can be “higher” or “lower,” but that’s about as specific as we often care to get.
Again: If it’s more of a sound than a note, it’s probably unpitched; for everything else, you can assume that it is pitched.
And if it has pitch, we need a way of indicating which specific pitch we want . . . .