My textbook states:
Metallic hydrides are formed by many d- and f-block elements. Unlike saline hydrides, they are almost always non-stoichiometric, being deficient in hydrogen, for example, LaH$_{2.87}$, YbH$_{2.55}$, etc.
After some research online, I found that non-stoichiometric compounds are so called because they don't follow the law of definite proportions, that is, "their elemental composition cannot be represented by a ratio of well-defined natural numbers."
From what I understand, some transition metals form hydrogen bonds with H atoms that fill interstitial spaces between metal atoms in crystal lattices. However, what does "hydrogen-deficient" mean in this context? Apologies if this is an elementary question, but I could not find an answer online.