Title case capitalises the significant words in a heading and leaves short function words in lowercase — the convention most newspapers and many style guides use for headlines.
Which words stay lowercase
Articles, short prepositions and conjunctions: a, an, the, of, in, and, or. The first and last words are always capitalised regardless of what they are.
Applying it consistently
Doing this by hand across a list guarantees inconsistency, because the rule about which words stay lowercase is exactly the kind of thing people apply differently on a Friday afternoon.
Style guides disagree
Different guides draw the line at different preposition lengths. Pick one convention and apply it to everything rather than mixing conventions within a publication.
Try it: Convert Text to Title Case on SeoWolf's Notepad.