Dagesh

A symbol, represented by a dot, placed in the centre of certain letters in the Hebrew writings (both BH and modern) to do two things basically.

1. Six letters of the Hebrew alphabet can have alternative sounds, a hard (plosive) or a soft (spirantized); these are collectively given the name begatkephat. These are:
בּ, גּ, דּ, כּ, פּ, תּ. A dot within them—the dagesh—indicates that they are to be read with a hard sound instead a soft one. E.g., ‎בָּם is to be pronunced bam instead of vam. The dagesh in these cases are called dagesh lene or dagesh qal (light/weak dagesh).

2. When used with any letter, except the gutturals, the dagesh indicates that the letter is to be doubled; so הִנֶּה = hin|neh and ‎אֶת־הַדְּבָרִים = ‘et-had|d^e|va|rim. When thus used, it is referred to as a dagesh forte or dagesh hazaq.

What looks like a dagesh also appears with some other letters, but are not true dagesh:

1) One of them is the vowel sign shuruq, ‎וּ, 'u.'

2) The other is a diacritical mark known as a mappiq, and is found with the letter ‎הּ, occasionally with א, and ו, and י, especially when it is the final syllable of the word. In this case the mappiq serves to indicate that it is not there to flag the reader how the syllable is to be voiced, that that it actually has grammatical value (hence, it is not a dagesh at all). Take the example of the word ‎וּמֵאִישָֽׁהּ (Ruth 1:5). A careless reader would read this word as u + me + ‘isha, "and from a woman." The mappiq instead alerts the reader to the fact that the letter is to be treated as a consonant (not a vowel) and that it is the 3rd feminine singular possessive pronominal suffix (i.e., her) → u + me + ‘ish + ah, "and from her man/husband." This use of the mappiq to indicate the suffix to mean "her" is quite common in BH.

Tim McNinch, "Hebrew Grammar - Dagesh," Blackboard Hebrew.
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Gesenius' Hebrew Grammar: select from the bookmarks the follow §12-14.