What is the significance of "boot" in a 1031 exchange?

In a 1031 exchange, "boot" refers to any non-like-kind property or cash received by the taxpayer. The significance of boot is that it may be subject to capital gains tax, whereas like-kind property exchanged in the transaction is typically tax-deferred. It is important for taxpayers to be aware of boot in order to properly structure their 1031 exchanges to minimize tax consequences.