A sheva is a letter which makes a vowel sound in Hebrew (also called a half vowel or very short vowel). It looks like an English colon and always stands beneath a letter as here:
לְ
Start here §2j. There are two kinds of shevas: video
A vocal sheva begins a syllable; a silent sheva ends a syllable; video
A vocal sheva is preceded by a long vowel; a silent sheva is preceded by a short vowel; video.
When two shevas occur together, the first is silent; second is vocal; video
Also:
Gutturals will not take a vocal sheva; it will shift to a composite sheva. They will take a silent sheva.
Letters with a sheva for the vowel will often lose their dagesh (assuming they had one); see the Coal Mine letters.
“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.
When two shevas occur on the beginning of a word, changes occur; video