I have a strand of LED light. Why wont my arduino board power them but they will power a group of LEDs
A bit confused on what you're asking.. but, I'm assuming you're talking about running a bunch of leds in series on the Arduino?
Which, is not a good idea. Unless you have external components, transistor or MOSFET, etc.
Anymore than 1 or 2 LED's from 1 pin (with the correct resistor) is not a good idea. The pin can only source a max of 40ma, most LED's take about 20ma. If you're putting a group of LED's (which is what it sounds like you're doing) there's a very good chance you're going to blow a pin, if not ruin the whole chip!
Look into using Transistors, relays, uh, FETs.. any of those should work for you.
If they are in series the forward voltages will likely sum up to more than 5V and they won't light at all. 1 or 2 in series is the limit with 5 volts. If its these new fangled white things, with 3v + forward voltage, one only.
Try putting them in parallel...
If it's one of those "ready made" LED strips, they usually require 12V, and much higher current than Arduino can supply.
You need an external powersupply and a transistor to drive those.