John Locke's most politically influential work is probably his two Treatises of Government. The first of which, is mostly written as refutation of a work titled Patriarcha, by Sir Robert Filmer, which was mostly written as a philosophical attempt to support the divine right of kings. Arguing for a hierarchical system in which kings, who are somehow considered to be the heirs of Adam, are closer to god than their subordinates, and as such, have the right to govern their subjects as a father has the right to govern his family.

The second treatise deals mostly with Locke's theory on why and what governments are.

https://www.gutenberg.org/files/7370/7370-h/7370-h.htm

The commonly touted political theory associated with John Locke is the 'Social contract'. He argues that governments are formed as a means of instituting a system of order that protects its citizens natural inalienable rights. (Mostly the classic, trio: Life, Liberty and property) As such, any system of government is a contract between its citizens, and the government itself - Citizens surrender some freedom in the interest of preventing anarchy and protecting their lives and property. In Locke's understanding, the Social contract can take many forms, such as a parliamentary democracy, or a constitutional monarchy and non of them are inherently superior. (Of course, his first treatise debunks the suitability of an absolute monarchy)

Generally speaking, I would argue that Locke doesn't believe that Democracy is essential. but, he does argue that if any Social Contract descends into a tyranny - where a government ignores, or violates the rights of its people - it is the duty of its populace to revolt, overthrow the government, and institute a new contract that respects the rights of its citizens. Although this may sound like an incitement to form a democracy, it doesn't in practice mean it. The Glorious Revolution of 1688 overthrew a sitting monarch and replaced him with a new constitutional monarch, (admittedly, the reasons for the "revolution" were entirely religious) replacing James II with William III safeguarded the rights of the protestant majority, but did so at the expense of the Catholic minority, the people merely switched a king with a different king.