Gideon said to God, “If you will save Israel by my hand as you have promised—look, I will place a wool fleece on the threshing floor. If there is dew only on the fleece and all the ground is dry, then I will know that you will save Israel by my hand, as you said.” And that is what happened. Gideon rose early the next day; he squeezed the fleece and wrung out the dew—a bowlful of water.
Then Gideon said to God, “Do not be angry with me. Let me make just one more request. Allow me one more test with the fleece, but this time make the fleece dry and let the ground be covered with dew.” That night God did so. Only the fleece was dry; all the ground was covered with dew. Judges 6:26-40

Gideon, a farmer, was asked by the Lord to attack the Midianites. They were the most frightening power in the land at the time. Being a simple man and asked to go up against such a strong army, Gideon wanted to make sure he heard from the Lord, so he asked for a sign and a confirmation of a sign.

Then he did what God asked him to do.

However the Bible seems to be against testing the Lord.

Do not put the LORD your God to the test as you did at Massah. Deuteronomy 6:16

Jesus answered him, “It is also written: ‘Do not put the Lord your God to the test.’ “ Matthew 4:7

These are different kinds of testing. The people in Massah and Satan did not “test” in order to receive confirmation, they argued in defiance. Gideon wanted confirmation so he could obey. Other "testers" only want to quarrel and never have any desire to obey.

Remember, Gideon, previous to the fleece incident, already obeyed the Lord and tore down the idols his people had constructed. He was obedient, but before such a great task as attacking the Midianites, Gideon wanted to make sure.

Testing here means “seeking confirmation.”