Over the last decade, the Atacama Large Millimeter/submillimeter Array (ALMA) made it possible to observe protoplanetary discs, the birth sites of planets, at unprecedented angular resolution and sensitivity, revolutionising our understanding of planet formation. When observed at sufficiently high angular resolution, protoplanetary discs most often display sequences of axisymmetric dark and bright substructures, colloquially referred to as "gaps and rings". The origin of these substructures and the role that they play in the planet formation process are, however, still debated: substructures are considered to be either the signposts of ongoing interactions between massive (proto-)planets and their hosting discs, or ideal locations for the formation of (new) planetary bodies. The best way to solve this "chicken and the egg" problem is characterising the physical properties of these gaps and rings. In my talk, I will first discuss some recent attempts to observationally infer the size, density, and temperature of dust in these rings, relying on modelling high-resolution multi-frequency (from (sub-)mm to cm wavelengths), continuum observations in a handful of well studied systems. In particular, I will focus on CI Tau, the youngest (and only T Tauri) star where the presence of a candidate hot Jupiter was proposed based on long term radial velocity monitoring. My high-angular resolution and sensitivity continuum observations revealed that in CI Tau the dust density and fraction of large grains locally peak at the position of the bright rings, suggesting that dust trapping is taking place, and that the growth of dust is limited by particle bouncing or fragmentation. Furthermore, my data were able, for the first time, to provide a characterisation of the bulk composition and structure of grains, suggesting that amorphous carbonaceous grains with <50% porosity best fit the data. I will then introduce a new technique that combines these dust properties with gas kinematics to understand if bright rings are prone to the formation of planetesimals under streaming instability. In the case of HD 163296, the only source where dust properties and gas kinematics have been both well studied so far, my method reveals that the outermost ring shows the right physical conditions for particle clumping to be triggered.