A TIN is a unique combination of letters and/or numbers assigned by tax authorities to identify individuals and entities for tax purposes. You will normally find it on any letters or correspondence you receive from your tax authority (the relevant tax authority in the jurisdiction(s) in which you are a tax resident). Some authorities do not issue a TIN, however, they will often use a similar type of number which can be known by a different name e.g.:
Unique Tax Reference (UTR) in the UK;
Social Security Number (SSN) for individuals and the Employer Identification Number (EIN) for most entities in the US;
SIREN in France; or
NIP/PESEL in Poland.
The OECD website can be consulted for further details about the TIN or its equivalent: www.oecd.org/tax/automatic-exchange/crsimplementation-and-assistance/tax-identification-numbers