See Types of Dagesh.
בּגּדּזּטּיּכּלּמּ נּסּפּצּקּרּשּתּ
The Dagesh Ḥazaq (Hebrew), “Strong Dagesh”, “Doubling Dagesh”, or Dagesh Forte (Latin, more commonly used in grammars) is used to indicate doubling (gemination) of a consonant, which also means that it form the ending of a closed syllable with the preceding consonant and vowel, in addition to acting as the beginning a new syllable.
Letters which can take a Dagesh Forte include any of the non-final (non-sofit) BGDKPRT letters (but rarely Resh, as it is often treated as a guttural), and most of the other (non-sofit) letters as well:
zeyin, tet, yod, lamed, mem, nun, samek, tsadi, qof, shin.
A Dagesh Forte can only exist in a medial position; it cannot begin or end a word.
When transliterating a letter with a Dagesh Forte, it should be written twice.
א ה ח ע ר
Guttural letters cannot take a Dagesh Forte. They cannot be doubled. The letter resh has some exceptions to this, as it isn't fully treated as a guttural.
Instead of takin a Dagesh, circumstances that would call for this cause compensatory lengthening to occur instead.