The small triangle is your current location, while the yellow circle that surrounds it is your scan range. The small glowing orbs are Exotic Matter (XM). Your current XM reserve is represented by the bar above your name. It is increased by getting an XM particle within your scan range.

Your primary goal is to collect XM. Deploy Resonators to create a new portals. Line those portals together to develop fields then hack the friendly portals for rewards. To work towards this goal, use your blasters to attack and destroy other faction's portals. After they are pinned down, you can steal them for your own faction.