As we know, in Arabic 3 letter roots make up the words. Different permutations of these roots make different words (i.e. adding and alif before the root or a shadda (ّ ) (which lengthens the sound of the letter) on the middle letter of the root changes the word). For example the word رَكَبَ (rakaba) a measure 1 verb means "to ride", while رَكّبَ (rakkaba) means "to install" (literally "to make something ride"). رَكّبَ is a measure 2 verb. Adding a shadda on the middle letter makes a verb measure 2.
You can use the measure of the word to help you with the meaning of the word. If we look back at our example we see that measure 2 takes the measure 1 meaning and does that action to something else. Measure 4 also does this. Measure 4 verbs have an alif in front of the 3 letter root. An example for measure 4 would be دَخَلَ (dakhala) which means "to enter" and أدْخَلَ (adkhala) which means "to make something enter" or "to bring something in".
Here's a link to a measure chart from fatwa-online. The chart is at the bottom of the page and titled "A set of 9 important Arabic Verb Charts". Also, if you are really serious about learning Arabic it's important for you to have a good dictionary. The best Arabic-English dictionary is the Hans Wehr. You look up a word by its 3 letter root and under that it will have a list of all the measures for that root and their meanings. I've gone through 2 copies of this dictionary in my time learning Arabic. I used the first one so much that the cover started to come off. Before long I had lost pages and couldn't look up anything that started with ي. That was when I decided it was time to buy a new dictionary.