A file stores bytes, and an encoding is the rule that maps those bytes back to characters. Two tools that use different rules read the same bytes as different text.
About 1 min of reading
Encode and decode text.
Encode a word to utf-8 bytes.
Before you run it, say out loud what it will print. Then open the answer.
Decode two bytes that mean an accent.
Before you run it, say out loud what it will print. Then open the answer.
Encode text with a combining accent.
Before you run it, say out loud what it will print. Then open the answer.
Round-trip with the same rule.
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.
utf-8 stores plain letters in one byte and everything else in more. It is the rule almost every modern file and web page uses, so it is the safe default.
Garbled characters are two tools disagreeing about the rule. The bytes are fine; the reading is wrong. Finding the rule that was used is the fix.
print("owl".encode("utf-8"))encode turns text into bytes using a rule. The bytes are what a file holds, and decode turns them back.
print(b"\xc3\xa9".decode("utf-8"))Those two bytes mean one accented letter under utf-8. Read under another rule they mean two different characters.
print("café".encode("utf-8"))Text with an accent needs more than one byte. A tool that assumes one byte per character reads it wrong.
print("ok".encode("utf-8").decode("utf-8"))Encoding and decoding with the same rule returns the original. The whole discipline is using one rule everywhere.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.