T = RC; The time it takes for a capacitor Cto charge to ~60% through resistor R
IC = BetaIB; relationship between transistor base current and collector current.
VBE ~= .7V, voltage drop from Base to Collector.
IE ~=IC; relationship between collector current and base current.
In this design, the capacitor charges through the 1M resistor with the time constant described above. Because the charging circuit has such high resistance ( 1 MEG ohm ) that signal is buffered by the two transistors. The current gain of the two trasistors is the Beta of a single transistor squared. The voltage at R3 is the capacitor voltage minus two VBE drops as described above. Current thru R3, and thus output current = V(R3)/R3. That's why decreasing R3 increases current thru the LED.