Resistive Plate Chambers (RPCs) are gaseous ionisation detectors that are employed by the Level-1 muon trigger system in the barrel region of the ATLAS muon spectrometer. The Level-1 muon trigger system selects muon candidates that are produced in proton-proton collisions at the Large Hadron Collider (LHC). Muon candidates are associated by the Level-1 system with the correct LHC bunch crossing and with one of the six transverse momentum thresholds. The RPCs are arranged in three concentric double layers and consist of approximately 3700 gas volumes, with a total surface of more than 4000 $m^{2}$. They operate in a toroidal magnetic field of approximately 0.5 T and provide up to 6 position measurements along the muon trajectory, with a space-time resolution of about 1 cm $\times$ 1 ns. This contribution will discuss performance of the RPC detector and Level-1 Muon Barrel trigger system using proton-proton collision data recorded by the ATLAS experiment in 2018 at a center-of-mass energy of 13 TeV.