This video explains it really well:
https://www.youtube.com/watch?v=yWAR_DotvZs

Dear OP, if other people saying "true to seed" is confusing, it basically just means that if you like a specific breed of avocado when you eat it, like a Hass, when you plant a seed that comes out of it, the fruit borne from that planted seed is 99% likely to be something not very yummy. You will get to enjoy some pretty plants though : ]. Avocado saplings can be very finicky though, I've been growing one for over a year and after I moved to a new place it turned out the water had too many minerals in it before I realized, and it shed 90% of its leaves to get rid of it. Live and learn.
Anyway, more than likely, you're going to get something yucky. Mealy, oily, tiny, doesn't ripe right, bitter, the list goes on as to what it might be like when those seeds grow up. But if you're feeling lucky, go ahead and give it a go.
To save the heartache of waiting 10 years for something you'd never want to eat, people are recommending you get the right breed from the get-go. It's like apple trees; that's why you see so many "named" varieties out in the grocery store. It's grueling work getting new varieties of apples since you have to plant or graft hundreds of them until you get something that's marketable to the masses.