It's easy to trigger some fusion in a D-T mix. As in, an enterprising high school student can do it on his tabletop with parts mostly scavenged from tube televisions.

The problem is that the fuel mass that undergoes fusion has a lot of mechanisms for energy loss, which mean that you need to continously apply a lot of energy into the system to keep it going.

"Ignition" refers to achieving conditions where the energy output of fusion matches the energy loss from the hot spot. In this situation, it is no longer necessary to feed in energy to keep the reaction going, so long as there is sufficient fuel.