sheva

A sheva is a vowel in Hebrew (although not a full vowel).  It looks like an English colon and always stands beneath a letter as here:

לְ

There are three kinds of shevas:   video

1. A vocal sheva which looks like a colon : and is preceded by a long vowel (or is under the first letter of the word); see here.
2. A silent sheva which also looks like a colon but is preceded by a short vowel ; see here.
3. A composite sheva which is a sheva joined with one of the other vowels;  see here.

Also:

1. Gutturals will not take a vocal sheva; it will shift to a composite sheva.  Gutturals will take a silent sheva.
2. Letters with a sheva for the vowel will often lose their dagesh (assuming they had one); see the Coal Mine letters.
3. “A silent sheva is usually placed in a final kaf in order to distinguish the latter from a final nun, e.g. הָלַךְ.”   Van der Merwe, A Biblical Hebrew Reference Grammar, 38.
4. When two shevas occur on the beginning of a word, changes occur; see here.