And thou shalt keep the feast of weeks unto the LORD thy God with a tribute of a freewill offering of thine hand, which thou shalt give unto the LORD thy God, according as the LORD thy God hath blessed thee:
You are to celebrate the Festival of Weeks to the Lord your God with a freewill offering that you give in proportion to how the Lord your God has blessed you.
Deuteronomy 16:10 In-Context
For six days eat unleavened bread and on the seventh day hold an assembly to the LORD your God and do no work.
Count off seven weeks from the time you begin to put the sickle to the standing grain.
Then celebrate the Festival of Weeks to the LORD your God by giving a freewill offering in proportion to the blessings the LORD your God has given you.
And rejoice before the LORD your God at the place he will choose as a dwelling for his Name—you, your sons and daughters, your male and female servants, the Levites in your towns, and the foreigners, the fatherless and the widows living among you.
Remember that you were slaves in Egypt, and follow carefully these decrees.