This is the 11th most frequent Spanish word.
lo
"Lo" is a neuter article or pronoun in Spanish used to refer to abstract ideas, concepts, or unknown things, often translated as "it" or "the" (e.g., "the thing").
Here, 'lo' is used as a neutral pronoun meaning 'what,' referring to an abstract idea or concept.
Lo que me preocupa es su bienestar.
What worries me is their well-being.
In this case, 'lo' functions as a direct object pronoun replacing 'him' or 'it' in the sentence.
No lo he visto desde ayer.
I haven't seen him since yesterday.
Here, 'lo' acts as a direct object pronoun referring to 'it,' the object of the action 'hacer' ('to do').