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).
SOURCES AND FURTHER READING
Khan, Geoffrey. 2020. The Tiberian Pronunciation Tradition of Biblical Hebrew. Cambridge: Cambridge and Open Book Publishers. §I.3.1.