The Propeller chip is a multicore microcontroller - it has eight processor cores inside that can run different pieces of code at the same time, sort of like having eight different brains working on individual tasks for a project.
When creating a Blocklyprop project, the main code you build runs in one processor. The BlocklyProp Functions and Multicore tutorial [1] explains more about how this works.
Throughout the Block Reference, look for this "brain" icon as an indicator that a block launches one or more cores automatically. See each block's reference for details
Links
[1] http://learn.parallax.com/tutorials/language/blocklyprop/blocklyprop-functions-and-multicore
[2] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/control
[3] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/servo
[4] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/robot
[5] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/sensor/colorpal
[6] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/sensor/gps
[7] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/sensor/sound-impact-sensor
[8] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/sensor/rfid
[9] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/communicate/protocols
[10] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/communicate/rgb-leds
[11] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/communicate/xbee
[12] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/communicate/wx-wifi/advanced-wx
[13] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/analog-pulses/voltage
[14] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/analog-pulses/pwm
[15] http://learn.parallax.com/support/reference/propeller-blocklyprop-block-reference/audio