Why mark targets?

[Bixby]

I was looking for someone to explain toe me these numbers... what does they actually means

The numbers are the party members' place in the enmity list for the currently targeted enemy. A has aggro, 2 is #2 in enmity, etc.

and what does this have to do ?
is it a sequence of targets , and for others not to mark?

That's the enemy target list, and I think it's a beta screenshot, since the red dots aren't shaped like that anymore. It's a measure of enmity for everything you have any enmity on at all. Small green/blue dot means you're nowhere near taking enmity. Yellow triangle means you're getting closer. Orange triangle means you've almost got enmity. Big red dot means you have enmity.