There is a lightning bolt icon flashing on the top right of my screen. What does it mean?
Here's a picture:
That's the new undervoltage symbol. Your Pi's power supply is marginal or insufficient for its requirements.
From the raspberrypi.org blog page:
Finally on the subject of icons, in the past if your Pi was working particularly hard, you might have noticed some yellow and red squares appearing in the top-right corner of the screen, which were indications of overtemperature or undervoltage. These have now been replaced with some new symbols that make it a bit more obvious what’s actually happening; there’s a lightning bolt for undervoltage, and a thermometer for overtemperature.
The lightning bolt means that the Pi is not receiving the required voltage. This may be due to the power supply you are using or the cable (if separate). You should switch to a better supply/cable as undervolting can lead to SD card corruption and other difficult-to-trace issues.
You don't mention what model Pi you are using and what if anything you have plugged into the USB ports, but you may be able to get by with your existing power supply/cable if you use a powered USB hub to connect any USB device.
The lightning bolt means that you don't have enough power going to the Raspberry Pi.
Almost any standard micro USB cable will be able to power the Pi. Most of us have one or two laying around from getting them for our phones and we could easily use what we have to charge our phones to power the RPi.
However, the Raspberry Pi should have a 5 volt 2 amp USB power cable to sufficently power the the device (and not get the bolt).
Most Android/Smartphone/USB chargers though run on a 5 volt 1 amp charger, and some of the "fast chargers" are only 5 volt and less than 2 amp. These are big generalities and are constantly changing and do vary depending on how cheap you got it and where from and for what purpose.
Some of those "fast chargers" can go for up to $30, and will be a 5 volt 2 amp charger. However, you can get one from adafruit.com for just about $8, which is what a regular phone charger would cost elsewhere, see 5V 2.4A Switching Power Supply w/ 20AWG 6' MicroUSB Cable.
This amount of power, might be too much for your computing needs for the Pi. So a standard 5 volt 1 amp charger might be good enough until you decide what you are doing with it or just while you are learning and goofing with the OS and such.
I bought a 5 V 2.1 A from somewhere and it works great for my Pi, similar to the one from Adafruit.com. I forget where I got it though.