The shadda, also called taʃdīd (تَشْديد), is a marker that is placed over consonants to indicate that the consonant is doubled. Within the English language, doubled consonants are quite common with some expressing a single sound ("stiff", "lesson", "normally") and others expressing a double sound ("misspell", "meanness", "roommate").
Arabic requires a double sound to be expressed when a shadda is used; no single sounds allowed. Consider the vocalization of 'red door' and 'redoor'. In the first case, there are two syllables and you will notice that the "d" sound is at the end of the first syllable and it is also at the start of the second syllable. The net result is a 'stretching' out of the consonant "d" which is what the shadda does. In the second case, there are also two syllables but there is no "d" sound in the first syllable. It only occurs in the second syllable, making the "d" sound seem shorter in duration. The key point is that the shadda doubles the consonant which 'stretches' the sound of the consonant.
Most Arabic verbs are composed of 3 letters (usually referred to as radicals). You will often come across a verb like بَرَّ which appears to be a two letter verb but actually is a 3 letter verb due to the shadda.
In the examples below, you will notice that short vowels are placed above or below the shadda to indicate the vowel sound that follows the 'stretched' consonant. Fatɧa and ɖamma are placed above the shadda. Kasra is generally placed below the shadda but above the consonant; but sometimes it can be seen below the consonant.