I do not understand how players opinions of you work. What can I do to raise their opinion of me, and what causes the players to dislike you?

Starting and getting significant playing time at a position they have a high overall rating at will increase their opinion.

Not starting or getting significant playing time at a position they have a high overall rating at will decrease their opinion (e.g., if a 100 rated TE is starting at K but not at his native position, he'll likely be unhappy, despite technically being a starter).

Trading and cutting players will also make them unhappy.

That helped, but one last question. What does this effect, and does it matter?

Unhappy and livid players will not re-sign with you. Happy players will take longer to drop to 'unhappy' status. So it really only matters in a 'contract' year, or if you cut/trade a player and later might want to pick them back up.

Ok, thank you.

I've a question about this, I saw a player that I traded on another team and when I looked at the roster it says he was 'livid' in opinion of you is that against me or his new owner?

He is livid towards you, not his new owner.

Ha ha brilliant, tbh I would have been too.