OTP stands for One True Pairing ("Um Par Verdadeiro", in Portuguese translation). This is slang used a lot in fandoms to refer to the characters that make up the perfect couple from each fan's point of view of the respective work (film, series, book, etc.).
In short, the OTP's are the fan favorite ships, that is, it consists of the fictional couple that the fanboy or fangirl more defends and wants it to have a "happy ending".
know more about shippar.
From the idea of OTP another abbreviation emerged in fandoms: O BROTP, joining "bromance" (brother + romance) and "one true pairing". This acronym is used to represent the friendship relationship that the fan most admires and hopes to work out in the accompanying work. The female version of BROTP is the SISTP ("sister" + "otp").
See also:meaning of Fandom.
Remember that while shippar and OTP are similar, they are also different. The ship turns out to be something more "broad", encompassing all the couples that the fan would like to have a romantic relationship. The OTP, as said, is the fan's favorite ship among all the others he may have in the same series or movie, for example.
See also:meaning of shippo.