grammar.articles.neuter-lo
The neuter article lo: lo bueno, lo de ayer
Lo + adjective builds abstractions ('the X thing'): lo importante, lo mejor. Lo de = 'the matter of'; lo que = 'what (the thing that)'.
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Explication
The neuter article lo has no plural and no gender; it combines with a masculine-singular adjective to build an abstraction — 'the … thing/part': lo importante ('the important thing'), lo difícil es empezar, lo mejor, hicimos lo posible. It's one of the most useful constructions in Spanish.
Lo + adjective/adverb + que is an intensifier meaning 'how …': no sabes lo caro que está todo ('you don't know how expensive everything is'), me sorprende lo rápido que aprende.
Two more patterns: lo de + noun/infinitive refers to a known matter or affair (¿resolviste lo del visado?, lo de ayer), and lo que introduces 'what' in the sense of 'the thing that': dime lo que piensas, haz lo que quieras.
Exemples
Cheap things end up expensive.
Région: global
The moving matter is still pending.
Région: global
The good thing is we arrived; the bad thing is we lost the luggage.