A transistor behaves like a switch (or a valve on a water pipe).
Electricity at b allows electrical flow from c to e.
Without electricity at b, there is no flow from c to e.
Therefore, you can CONTROL the flow of electricity. Hence, ON/OFF switch.
The transistor shown is discrete. You can buy one for a few Baht at “Pot House”. (บ้านหม้อ)