Any Hebrew letter (except the gutturals) can have a dot inside of it called a "dagesh mark" (dagesh means "emphasis"). There are basically two kinds of dageshim that you need to know: the dagesh lene (kal) and the dagesh forte (chazak).
The Dagesh Lene (Kal)
Six Hebrew letters you have studied, namely, Bet, Gimmel, Dalet, Kaf, Pey, and Tav may appear with or without a dot placed within them. This dot is called a "Dagesh Lene." If one of these six letters has a Dagesh Lene mark it will have a hard pronunciation, otherwise it has a softer pronunciation.
Collectively these letters are sometimes called "Begedkephat letters" as an acronym for the names of letters:
The Dagesh Forte (Chazak)
All of the Hebrew letters (except the gutturals) - including the Begedkephat letters - can take a dot that looks exactly like the Dagesh Lene but is called a Dagesh Forte. The dagesh forte can appear in ANY Hebrew consonant except a guttural letter: