The terms transmission and distribution refer to two different parts of the infrastructure (the grid) which moves electricity from centralised power plants out to end-users.
Transmission refers to the central part of this infrastructure, including the long-distance, high-voltage power lines (often 100’s to 1000’s of kilovolts) that move electricity around countries or large regions.
Distribution refers to the local part of the grid: the lower-voltage power lines (10’s of kV down to household 120/240V) which spread electricity supply out to, eventually, individual premises.
In practice there is no all-encompassing definition of when one ends and the other begins. Voltage levels can differ in different countries, changes in operating entities may provide a more appropriate definition of the transition; and so on.
Broadly-speaking though, the transmission part of the grid tends to be “smart”, with extensive real-time data collection and analysis enabling the two-way power flows and signalling controls required to ensure that the many connected power plants are always balanced with demand in an optimal manner. Whereas the distribution part of the grid is often characterised by a radial network topology with largely one-way power flow from the grid centre out to end-users.