These two letters are silent. They don’t make any sound by themselves.
ъ – hard sign
ь – soft sign
First of all, both of those signs work as separation signs. In order to understand the difference you should listen to the following words, using Google Translate:
сел        лёд
съел      льёт

In addition to the separation job soft sign does something else – it softens the previous consonant:
мол     тонко
моль    Тонька