Teaching English to Speakers of Other Languages.
The abbreviation refers to the certification of English teachers, which includes both TEFL and TESL. In other words:
That is the presence of a TESOL (or TEFL/TESOL) certificate indicates that the teacher has been trained to teach English as a foreign language (TEFL) and a second language (TESL).
Our educational center periodically makes discounts up to 55% for the TEFL/TESOL courses, at the end of which a certificate of the same name is issued that does not have an expiration date (its authenticity can always be checked here).