555 timer explanation
You need to consider this in context of the full statement


  The output is fairly powerful, especially when the supply voltage is 15V, and will supply up to 200 mA. Even at 5V, the 555 will easily drive LED's and small relays.


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  Why is this true?


The output transistors of the 555 have a relatively large current handling capability. 200mA output at 5V is a full Watt, while at 15V, that is 3W. That's a good bit of output, compared to a microcontroller.


  Why is this important?


If you want to drive LEDs or relays directly, this tells you that the 555 is capable of driving moderate loads without external components or external buffers. This is useful if you want to keep your parts count low or would rather not deal with a buffer stage.