  By means of density functional theory plus dynamical mean-field theory
(DFT+DMFT), we investigate the correlated electronic structures of
La$_3$Ni$_2$O$_7$ under high pressure. Our calculations show that
La$_3$Ni$_2$O$_7$ is a multi-orbital Hund metal. Both the 3$d_{z^2}$ and
3$d_{x^2 - y^2}$ orbitals of Ni are close to be half filled and contribute the
bands across the Fermi level. Band renormalization and orbital selective
electronic correlation are observed. Through imaginary-time correlation
functions, the discovery of high-spin configuration, spin-frozen phase, and
spin-orbital separation shows that the system is in a frozen moment phase at
high temperatures above 290 K and is a Fermi liquid at low temperatures, which
is further comfirmed by the calculated spin, orbital, and charge
susceptibilities under high temperatures. Our study uncovers Hundness in
La$_3$Ni$_2$O$_7$ under high pressure.
