Every letter is a number, and a file stores those numbers as bytes. The mapping from number to letter is a table, and the rule for reading it is the encoding.
About 2 min of reading
Inspect how text becomes numbers.
Print the number for A.
Before you run it, say out loud what it will print. Then open the answer.
Print the letter for 65.
Before you run it, say out loud what it will print. Then open the answer.
Show a word as numbers.
Before you run it, say out loud what it will print. Then open the answer.
Compare character count and byte count.
Before you run it, say out loud what it will print. Then open the answer.
Read the first example one line at a time. A line that prints shows what appears on screen; the rest set things up quietly.
Every program that says utf-8 uses the same table, so text written on one machine reads on another. The agreement is the whole point of the standard.
A string length counts characters. A file size counts bytes. They are equal only for plain letters, which is the common source of off-by-one bugs.
print(ord("A"))
print(ord("a"))A is 65 and a is 97. Letters are numbers by a table, and the table is what an encoding defines.
print(chr(65))The number 65 prints as A. The mapping goes both ways, which is how text is stored and read.
print([ord(c) for c in "owl"])Three letters are three numbers. A file holds the bytes of those numbers, nothing else.
print(ord("é"))An accented letter is 233, which needs more than seven bits. Plain letters fit in one byte, most other characters do not.
print(hex(ord("é")))The number for a character is its code point. The number is fixed worldwide, the bytes that store it depend on the rule.
print("é".encode("utf-8"))
print("é".encode("utf-16"))The same character becomes two different byte rows under two rules. The rule used has to be the rule read.
print("owl".encode("utf-8").decode("latin-1"))Plain letters are the same bytes in the common rules, so they survive a mismatch. Everything else can change.
s = "caf\u00e9"
print(len(s), len(s.encode("utf-8")))Four characters take five bytes. Character count and byte count differ once accents appear.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.