An IBAN looks like a string of letters and digits, but every character has a specific job. Mistype one and a transfer can bounce, or land in the wrong account. The check digits built into every IBAN are there to catch that.
What an IBAN contains
Every IBAN starts with a two-letter country code, then two check digits, then the BBAN (Basic Bank Account Number). For Germany: country code DE, two check digits, an eight-digit bank code (BLZ), and a ten-digit account number, giving a total of 22 characters. Other countries have different layouts, from 15 characters (Norway) to 34 (Malta).
How the check digits work
The two digits after the country code are a checksum over the whole IBAN. To calculate them: rearrange the IBAN by moving the first four characters to the end, replace each letter with its numeric equivalent, then divide the resulting long number by 97. A remainder of 1 means the IBAN is internally consistent. Any error in any character changes the remainder.
This catches transpositions, single-digit errors, and most other typos. It does not tell you whether the account is open, only whether the number is well-formed.
What to do with the result
Paste the IBAN into the IBAN validator. You get valid or invalid, the country, the expected length, and (if valid) a breakdown of the structure. German IBANs also return the bank name and BIC, which is useful if you want to double-check you have the right bank.
If the tool returns invalid, look for spaces, a wrong country code, or a transcription error. Copying directly from your bank’s portal avoids most of these.
What a valid result does not tell you
Valid means no typo. It does not mean the account is open, that the account holder name matches, or that the destination bank accepts international transfers. For that, you need confirmation from your bank or the recipient.
TL;DR: Run your IBAN through the IBAN validator. Valid means no typo; it does not mean the account exists.