Faith is a principle of action. Faith is belief unto action, or what we are willing to do because of what we believe. Faith is the ratio of how much you are willing to do to how small a reason you have to believe. Your ability to believe things that are not readily apparent will dictate what and how much you can receive from God.
Unbelief is when you do not believe things when given sufficient reason to. Unbelief will limit your actions, which will in turn limit what God can give you.
You can only receive what God wills to give you up to the level of interest and effort you exercise:
9 And now Alma began to expound these things unto him, saying: It is given unto many to know the mysteries of God; nevertheless they are laid under a strict command that they shall not impart only according to the portion of his word which he doth grant unto the children of men, according to the heed and diligence which they give unto him.
10 And therefore, he that will harden his heart, the same receiveth the lesser portion of the word; and he that will not harden his heart, to him is given the greater portion of the word, until it is given unto him to know the mysteries of God until he know them in full.
11 And they that will harden their hearts, to them is given the lesser portion of the word until they know nothing concerning his mysteries; and then they are taken captive by the devil, and led by his will down to destruction. Now this is what is meant by the chains of hell. (Alma 12)
Unbelief is not an all-or-nothing affair. Rather than being like a light switch, it is more like a fuel gauge. Unbelief happens by degrees, just as faith—its opposite—does.
Unbelief is manifest in many ways, each of which is further described below.