How does Google Adds know so much?

[OliveSales]

I have conversations with friends and family all the time regarding the most random subjects. Once we had a conversation about a show called 'shameless' and my friends tried to get me to watch it. Couple hours later when I was browsing through Instagram I started seeing Ad's for Shameless.

I've had plenty of other times where just a conversation about something had Ad's start targeting me.

I think it's insane how this practice can be legal.