Baybayin is a writing system native to the Philippines, attested from before Spanish colonization through to at least the eighteenth century. The word baybay means to spell in Tagalog, which was the language most frequently written with the baybayin script.
It is one of a number of individual writing systems used in Southeast Asia, nearly all of which are abugidas, or alphasyllabary, where any consonant is pronounced with the inherent vowel a following it, and diacritics are used to express other vowels. Many of these writing systems descended from ancient alphabets used in India over 2000 years ago.
Baybayin is an abugida: that is, a writing system whose segments are based on symbols for consonants, with vowel notation a secondary part of the consonant-vowel unit. Baybayin thus resembles other Indic scripts such as Devanagari (used for Hindi), Tamil, and Javanese (all of which are derived from the Brahmi script of ancient India). The 14 consonant characters end in an a sound when unmarked.
To indicate an e or i sound, a kudlit (dot) is placed above the letter, while for an o or u sound a kudlit is placed below. Baybayin is read from left to right, though some early examples may have used a top-to-bottom order.
In contrast to English, when you write and read Filipino words, you just write and read every letter you see and/or hear. There are no hidden or silent letters or a need for denoting intonations; you just have to write and read as-is. Although, each letter and sound has to have the proper emphasis when you're speaking it.
Baybayin is so strongly associated with Tagalog because of the modern discourse of Tagalog regionalism and Filipino nationalism. There is a great deal of interest today in the revitalization of Baybayin, including many online resources. In some ways, Baybayin has been reclaimed as a symbol of Filipino national identity.
Passports issued by the government include Proverbs 14:34 in Tagalog, in both Latin (ang katuwiran ay nagpapadakila sa isang bansa) and Baybayin. The new series of banknotes includes the word Pilipino in bottom right in Baybayin. Furthermore, several bills have been filed in Congress to mandate the teaching of Baybayin and promote the script being used in other ways; there has been significant pushback, though, with some calling out the bills as promoting a nationalist agenda.