The tāʾ marbūʈa is a special letter that always goes on the end of a noun or adjective (not verbs) to indicate that the noun or adjective is feminine as opposed to being masculine. There are a few exceptions to this where the noun with a tāʾ marbūʈa on its end is actually masculine but this need not concern us at this point. It should be noted here that Arabic has only two genders (masculine and feminine). There is no neuter.
The third letter of the Arabic alphabet is the letter ت (taa). This ت is called tāʾ maftuɧa "the open taa". A tāʾ marbūʈa is a special version of the same letter ت. The word marbūʈa means "tied". It is as if the two ends of the ت have been brought together and tied up on top resulting in a "tied taa".
The tāʾ marbūʈa is only connected to the end of a word. If the word ends with و ز ر ذ د ا then the tāʾ marbūʈa sits on its own as ة. (Remember from the alphabet section where these 6 letters don't allow for a letter to be attached after them; only before them) For all the other letters, the attached tāʾ marbūʈa looks like ــة.
The tāʾ marbūʈa by itself is not pronounced in current spoken Arabic. It is the fatɧa short vowel that preceeds it that is pronounced. (See vowel section for understanding what fatɧa is) The end result is an "a" sound at the end of feminine words. In Classical/Koranic Arabic, the tāʾ marbūʈa is pronounced as a "t". This, combined with the preceeding fatha, produces an "at" sound at the end of the feminine word. There will be more on this later in the nunation section.