In a ceramic package, thermal vias are punched / drilled &
refilled with metal to improve thermal conductivity. They
may, or may not, be used for an electrical path as well.
It is an alternative to a heat-slug design which can be
more of a stretch to design / qualify. Thermal vias can
be a couple-of-layers mod to an existing package layup.
These vias pass through the body, while normal vias are
captive (across inner layers, but not exposed outside).
Internal vias have no impact to hermeticity and so on,
tend to be smaller and use the internal tungsten metal
rather than something like CuW (better for thermals).