The units involved in this conversion are pounds and kilograms. This is how they are defined:

The pound or pound-mass (abbreviations: lb, lbm, lbm, ℔) is a unit of mass with several definitions. Nowadays, the most common is the international avoirdupois pound which is legally defined as exactly 0.45359237 kilograms. A pound is equal to 16 ounces.

The kilogram (or kilogramme, SI symbol: kg), also known as the kilo, is the fundamental unit of mass in the International System of Units. Defined as being equal to the mass of the International Prototype Kilogram (IPK), that is almost exactly equal to the mass of one liter of water. The kilogram is the only SI base unit using an SI prefix ("kilo", symbol "k") as part of its name. The stability of kilogram is really important, for four of the seven fundamental units in the SI system are defined relative to it.

How much is 143 pounds in other mass units?