  Lanthanum hydride LaH$_{10}$ with a sodalitelike clathrate structure was
experimentally realized to exhibit a room-temperature superconductivity under
megabar pressures. Based on first-principles calculations, we reveal that the
metal framework of La atoms has the excess electrons at interstitial regions.
Such anionic electrons are easily captured to form a stable clathrate structure
of H cages. We thus propose that the charge transfer from La to H atoms is
mostly driven by the electride property of the La framework. Further, the
interaction between La atoms and H cages induces a delocalization of La-5$p$
semicore states to hybridize with H-1$s$ state. Consequently, the bonding
nature between La atoms and H cages is characterized as a mixture of ionic and
covalent. Our findings demonstrate that anionic and semicore electrons play
important roles in stabilizing clathrate H cages in LaH$_{10}$, which can be
broadly applicable to other high-pressure rare-earth hydrides with clathrate
structures.
