The Symbols are mysterious iconography used by the Army of the 12 Monkeys, which also appear in visions to Primaries, such as Jennifer Goines. Their meaning is as-yet unknown.
Given that their leader the Witness (Athan Cole) is himself a Primary, the symbols appeared to him in visions as well - and as he later revealed to Jennifer, he himself doesn't actually know what they mean.
One symbol is prominently used on the Amulet carried by the Army's leading members.
Another set appears in the circular clockface design that decorates the floor in some of their installations. These actually may or may not be part of the symbols, as the TV showrunners confirmed that they are Aramaic (apparently just numbering the clock, though why Aramaic was used is unknown).[1]
The full set of symbols are a sprawling design filling pages and pages of Jennifer's vision-based drawings.