In simple cathode ray oscilloscope, electrons are emitted from the cathode and move towards the anode (screen).

There is a large potential difference, V set up between the anode and the cathode (with power supply) with the cathode at higher electric potential than the anode.

[Potential difference, V = U/Q                                                             where U is the electric potential energy, Q is the charge of electron in this case]

An electron at the cathode will have large electric potential energy, U (due to the large potential difference across the cathode and anode). As this electron moves towards the anode, its electric potential energy changes gradually into kinetic energy. [electric potential energy decreases as kinetic energy increases]. So, the electron is accelerated.