In the wilderness He fed you manna which your fathers did not know, that He might humble you and that He might test you, to do good for you in the end. (See also Exodus 16:4)
The test was whether and how they would receive it. If they were hungry enough, they would eat anything, but would they continue to expect it day after day as something good from God? Or would they abuse the privilege by greedily (and foolishly) hoarding it? Would they keep giving thanks for their daily bread, or begin grumbling about its monotony?
We often think of tests in terms of trials or feats of obedience, wondering, "What hard thing is God going to ask of me today?" But in this and other instances, God tests His people by giving them gifts -- whether they be the gift of life, the gift of a child, or the gift of His Son.
How have we responded to God's gifts?