CONSONANT GEMINATION [CC] or [Cː]
When dagesh is written in a letter and a vowel precedes it, the consonant should be held for a longer duration. For those familiar with Arabic, it has the same effect as shadda ( ّ ).
In the case of the consonants בג״ד כפ״ת, when not preceded by a vowel, the dagesh indicates that they should be pronounced with greater pressure (i.e., their "heavy" stop/plosive realization instead of their "soft" fricative realization).
dagesh makes a letter heavy and rafe makes it soft, as in גָּגֿ ‘roof’
Translation from Khan (TPTBH II.L.1.3.2)
אלדגש יתקל אלחרף ואלרפי ירכיה כק̇ גָגֿ
Text from Khan (TPTBH II.L.1.3.2)
SOURCES AND FURTHER READING
Khan, Geoffrey. 2020. The Tiberian Pronunciation Tradition of Biblical Hebrew. Cambridge: Cambridge and Open Book Publishers. §I.3.1.