Advances with "slashed circles" are OPTIONAL for advancement to the next era or age of technologies. Before you can start building "Middle Ages" techs, you need to finish all "Ancient Ages" techs ... EXCEPT for the optional (circle with a slash) ones.
If you've done all the "non-slashed circle" techs (ie all of the required ones) in the Ancient era, you CAN start building the next era ones (Middle Ages). However, some of the optional ones may be worth picking up depending on your play style. For instance, Ancient era "Literature" is an optional, but without having researched it you cannot build Libraries. Which can hurt your civs cultural rating a lot, as Libraries allow increases in both culture; AND in the science rate in cities where they are built.
This applies to all eras- all have optional ones not needed to start the next one.
If you go to the next era and leave some unbuilt, you can still go "back" to the previous era to select them to be built, using the small buttons at the bottom left and right on the Science advisor screen. Thats also how you move up to the next era when required ones are done but optional ones remain.
