As D- mesons are called mesons, which contain exactly one charm quark heaviest quark. Your rest mass is in the range of about 1.9 GeV / c ² and they live on average some 10-13 seconds.
D mesons are test particles for studying the quark- conversion via the weak interaction.
Similar to kaons and B mesons mix and neutral D mesons, so they can pass into their own antiparticles. This is examined, among other things, to find evidence of CP violation and physics beyond the Standard Model.