To put it simply two-level systems (TLS) are groups of atoms in a disordered solid that present two stable configurations, that is two physical states of similar potential energy separated by a small energy barrier. The barriers need to be small for it to be overcome by thermal fluctuation alone and thus for the TLS to resonate at a frequency close to the frequency of interest.
"You throw your sword at the dragon. Unfortunately the blade hits the dragon's
leg instead of its chest. The dragon howls in pain and smacks his foot against
your head sending you flying backwards
> Check my pockets
You find some coins in your pocket. They're not much but they'll buy you enough
time to run away. "
To put it simply two-level systems (TLS) are groups of atoms in a disordered solid that present two stable configurations, that is two physical states of similar potential energy separated by a small energy barrier. The barriers need to be small for it to be overcome by thermal fluctuation alone and thus for the TLS to resonate at a frequency close to the frequency of interest.
If you are interested by what exactly are these TLSs at the atomic level, have a look at this paper: https://journals.aps.org/prb/pdf/10.1103/PhysRevB.97.014201