SSH -2 single coil pickup and a humbucker
SSS -3 single coil (a classic example is the fender stratocastor)
HH- Double humbucker( ala les paul)
Ususlly the first letter means the neck pickup(Closest to guitar neck), 2nd letter is the middle pickup while the 3 letter is the bridge pickup. All 3 will have different characteristic due to the placement of it in relative to the string tension of the guitar.
Tone characteristic, single coil PU, when compared to Humbucker, tend to be thinner and more "glassy"(difficult to put in words, you will need to hear it). Classic example of single coil clean tone wud be stuff from SRV to Eric clapton to eric johnson and to some of red hot chilli pepper guitar tone.
Humbucker- Tend to be fatter and muscular in tone. Example, slash, Joe satriani etc.
Function of the pickup?
Basically and in the easy way to put it, its a piece of magnet wilh copper wire coil round and round it. When put in a guitar, it will pick up the string vibration, convert into the guitar sound that we hear thru amplifier.
For a newbie, imho, it dont matter so much as the ear is still untrained to differenciate between treble response, mid level, output "hotness", clean response, overdrive/distortion. Put it simply, to know difference in tone. Stock PU that comes with the electric guitar will do for the time being. Those after market PU are only suitable when you know what specific tone or characteristic of guitar tone you want. If not, it wont make a difference.
For a first electric guitar, my view is that, learn the basic well, etc chords changing, power chord, barre chord, sense of rhythm, melody. The pursue of the "best of gear" is a joke when the skill is zero.
Budget for PU, depend on how much you can spend. From 2nd hand pick up to brand new ones can ranged from $80-$400++. Yes, there are some PU that can get you a brand new electric guitars. So if you are not sure on the tone you are after, stick to the stock PU and, practice, practice and more practice