Okay kiddo, so you know how if you go to see an orchestra play music, there are lots of different instruments all playing together to make beautiful music? Well sometimes, instead of writing out the whole name of each instrument, people use special short names called shorthand.
Let's start with the strings section. The violins are the highest-sounding ones, and they're usually split up into two groups: the first violins (which play the higher notes) and the second violins (which play lower notes than the first violins). Violas are a little bit bigger than violins and they sound a little lower, and cellos are bigger than violas and sound even lower. And then there's the double bass, which is the biggest string instrument and plays the lowest notes of all.
Now let's talk about the woodwind section. These are the instruments that you blow into, like flutes, oboes, clarinets, and bassoons. Sometimes shorthand just says "woodwinds" instead of listing each instrument separately.
The brass section includes the trumpets (which are bright and loud sounding), French horns (which are big and kind of round-looking), trombones (which can slide up and down to create cool sounds), and tuba (which is the lowest and biggest brass instrument).
Percussion instruments also have their own shorthand. This includes things like the timpani (which are big drums that you play with sticks), snare drum (which is a smaller but still loud drum), bass drum (which is even bigger and lower than the other drums), and cymbals (which make a loud crashing sound when you hit them together).
So when people write shorthand for orchestra instrumentation, they might write something like "2 fl, 2 ob, 2 cl, 2 bsn, 4 hn, 3 tpt, 3 tbn, tba, timp, perc, hp, str" - which means two flutes, two oboes, two clarinets, two bassoons, four French horns, three trumpets, three trombones, one tuba, timpani, percussion (which could include anything from cymbals to a triangle to a snare drum), harp, and strings (violin, viola, cello, double bass).