When the rules of grammar would indicate that one of these letters would normally take a dagesh forte, they reject the dagesh forte and cause compensatory lengthening of the vowel in the preceding syllable instead.
For instance, the word for the earth or ground, “eretz”, starts with alef. If we want to add the definite article to it, which is normally heh followed by a short a vowel, since the alef rejects the dagesh, the heh instead gets a long a vowel. In this case, it also lengthens the vowel on the alef itself, so instead of haEretz the word becomes ha-Aretz.