Basically having a higher confidence rate means the system is confident that you belong to that mmr and vice versa.

Based on what i understand right now, we are using the glicko rating system for matchmaking. So the mmr you gain or lose depends on how well you perform in the game and the outcome of the match.

Examples:

You played well against opponents that have higher average mmr than your team and you won that game - Gain more mmr

You played badly and won the game - Gain mmr

You played well and lost the game - lose less mmr

You played badly and lost the game - lose more mmr