For example, if you have a pulse rifle with 290 light and you find another primary weapon with 300 light, you can infuse the new weapon into the old weapon and the old pulse rifle will now be 300 light. You can re-roll exotic armor using Glass Needles, which are purchased from Xur. The current max level is 40, and max light level is 400. Each light level still increases your damage, but it isn't as much as under the old system. You should do about the same damage as before. Now you are likely level 34 with a light level somewhere around 160 - 170. You were probably level 20 / light level 34 when you stopped playing. It is one screen to the left of your gear.
To summarize the bigger changes I can remember off hand: The Returning Players Guide on Reddit is probably a good place to start.
