What is the meaning of the law described in Leviticus 27?

Leviticus 27:29 is part of a section in the law that begins in v.14:

Lev. 27:14 ‘Now if a man consecrates his house as holy to the LORD, then the priest shall value it  as either good or bad; as the priest values it, so it shall stand.
Lev. 27:15 ‘Yet if the one who consecrates it should wish to redeem his house, then he shall add one-fifth of your valuation price to it, so that it may be his.
 

The topic in this section is a home or field consecrated to the Lord (i.e., given to the priests for their use). If the owner of that property later wants to redeem it (i.e., buy it back from the priests), then he must pay the price the priests have determined the property is worth according to the law. For example, the Lord says:

Lev. 27:19 ‘If the one who consecrates it should ever wish to redeem the field, then he shall add one-fifth of your valuation price to it, so that it may pass to him.
Lev. 27:20 ‘Yet if he will not redeem the field,  but has sold the field to another man, it may no longer be redeemed;
Lev. 27:21 and when it  reverts in the jubilee, the field shall be holy to the LORD, like a field  set apart;  it shall be for the priest as his  property.
Lev. 27:22 ‘Or if he consecrates to the LORD a field which he has bought, which is not a part of the field of his own  property,
Lev. 27:23 then the priest shall calculate for  him the amount of your valuation up to the year of jubilee; and he shall on that day give your valuation as holy to the LORD.
 

We see that the law established strict rules for how property set apart for the Lord could be transferred. Then in v.29, the Lord says that a man attempting to redeem property set apart for the Lord cannot use worthless things to pay the ransom. For example, the Lord says that a first born animal may not be used to pay the ransom, since all first born animals already belong to the Lord:

Lev. 27:25 ‘Every valuation of yours, moreover, shall be after  the shekel of the sanctuary. The shekel shall be twenty gerahs.
Lev. 27:26  ‘However, a firstborn among animals, which as a firstborn belongs to the LORD, no man may consecrate it; whether ox or sheep, it is the LORD’S.
 

Furthermore, a slave or any person already committed to the Lord under a vow (whether for good reasons or bad) cannot be used to ransom the property.

Lev. 27:28 ‘Nevertheless,  anything which a man  sets apart to the LORD out of all that he has, of man or animal or of the fields of his own property, shall not be sold or redeemed. Anything  devoted to destruction is most holy to the LORD.
Lev. 27:29 ‘No  one who may have been  set apart among men shall be ransomed; he shall surely be put to death.
 

For example, a slave who has been previously consecrated to the Lord is already the Lord's property, so such a slave could not be used as payment for the ransom. Likewise, a man already set apart to die for murder can be used to redeem property. That is the point of v.29.

Scripture quotations taken from the (NASB®) New American Standard Bible®, Copyright © 1995, 2020 by The Lockman Foundation. Used by permission. All rights reserved. www.lockman.org