There are two kinds of dageshim:  more here or this video

dagesh lene    video
dagesh forte    video
A dagesh lene makes the pronunciation of a BGDKFT letter hard; e.g. a ב is pronounced “v”.  A בּ is pronounced “b”. A dagesh forte doubles the pronunciation of a letter.
Only occurs in BGDKFT letters. Never occurs in a guttural.
Never preceded by a full vowel. Always preceded by a full vowel.
Note #1: letters with a sheva for their vowel will often lose the dagesh.
Note #2: the dagesh in a vav-conversive will never appear in the first letter of the verb root.