If you would like a second portion of dinner, please form a line behind the "seconds queue" sign hanging from the ceiling at 7:00 pm, and wait there until the server calls you forward.
Otherwise, it can be difficult for people coming into the dining hall for "firsts" to know where they should queue. And of course, everyone should have the chance to get a first portion before anybody takes a second portion. So that's why we have a separate line, clearly distinguishable from the "firsts" queue.
Seconds should usually be available from around 7:05 pm, as the kitchen staff have to weigh how much is left over at 7:00 pm before they offer anything for seconds. So that's why you should wait until you are called forward.