what does the "lo" in lo siento mean? does the word "lo" have a meaning by itself? Are there any other examples of when you would use "lo"?
"Lo" is "it". Lo siento = I regret it, I'm sorry, I feel (bad) about it.
You'll want to look up information about direct object pronouns in Spanish to learn more about this.
Here's one good resource: https://www.spanishdict.com/guide/direct-object-pronouns-in-spanish
Lo veo (I see him or I see it). Lo siento (I feel sorry about it). Lo siento (I feel it [speaking about a sensation or, in some places, about a sound])