Overwatch – What Does a White/Yellow Rank Icon Mean?

A blue chevron indicates a teammate that is not currently "in combat".

A hero becomes "in combat" when they damage the enemy or are damaged by them. In this case the chevron turns orange. I have witnessed orange chevrons over green-outline teammates multiple times when playing halers, so it is not tied to health.

Lastly, a green chevron indicates someone in your party.

The number signifies the amount of "Achievement Points" the achievement is worth. The amount of points directly corresponds with the Achievement's difficulty.

As of right now I cannot find a way to see all achievement points accrued. I don't like the current achievement menu; it shows you your 6 most recently earned achievements but you have no way of reading a description for them unless you find them on the drop down list. I would say achievement points are mainly for personal satisfaction; think of them as a reward for doing certain things. Some people may not feel like they have 'completed' a game until all achievements are unlocked. They give players something to use as a goal.